How does Christianity define "character"?

Let's break character out into 3 pieces:

1. Axioms - what are the fundamental beliefs that direct my decision-making, priorities, and aspirations?

2. Nature - what do I want? Paul argues that it is possible through the Holy Spirit for a person's nature to be changed (e.g. Romans 8:1-4. For those willing to use a Book of Mormon citation, see also Mosiah 5:2)

3. Priorities - I probably want more than one thing. How do I sort those priorities? If I have a desire/commitment to X, is it trumped by some other desire/commitment?

I do not see character as a physical thing; it is a shorthand to describe the fundamental desires & beliefs that guide a person's actions. Since all of the items cited can be changed, a person's character can develop. A dissertation on character could add mountains of detail to each of these categories, and add many subcategories to them.

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Christian teaching

I am unaware of any specific Christian doctrinal definition of character--the categories listed above could be used in non-Christian circles.

However, the scriptures do offer some clear examples pertaining to these categories and the kind of person God wants us to become:

Axiom example:

37 Jesus said unto him, Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind.

38 This is the first and great commandment.

39 And the second is like unto it, Thou shalt love thy neighbour as thyself. (Matt 22:37-39)

All the other commandments follow if this is treated as the object of our decisions.

Nature example:

Knowing this, that our old man is crucified with him, that the body of sin might be destroyed, that henceforth we should not serve sin. (Romans 6:6)

Our sinful desires can be overcome and replaced (though this is not meant to be easy).

Priorities example:

Saying, Father, if thou be willing, remove this cup from me: nevertheless not my will, but thine, be done. (Luke 22:42)

Jesus prioritized His Father's will over the desire not to suffer.

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Physics

The classic argument that free will cannot be real because it is incompatible with a Newtonian universe suffers from the minor handicap that we do not live in a Newtonian universe.

Reducing character (and thoughts, priorities, beliefs) to neurology would be to abandon a mind-brain distinction in favor of metaphysical naturalism. Aside from the fact that there is great reason to believe that the mind is not the brain nor a mere product of the brain, Christians who believe in the supernatural on other grounds would have no reason to appeal to naturalistic explanations of the mind in an effort to escape evidence for the existence of the soul.

Conclusion

Character is a description of attributes possessed by intelligent agents; though it may be influenced by experience, it is not deterministic.

Character has been defined as doing the right thing even when no one is looking.

Of course that definition doesn’t work in any religious setting where an all knowing God sits observing every deed done by every human.

Christianity tends to define character as doing what is right as a result of cultivating the fruit of the spirit. The scriptures say love of God and love of neighbor when practiced fully will be the best demonstration of good character.

Matthew 22:36-40 NWT

"Teacher, which is the greatest commandment in the Law?" 37 He said to him: " You must love Jehovah your God with your whole heart and with your whole soul and with your whole mind.38 This is the greatest and first commandment. 39 The second, like it, is this: 'You must love your neighbor as yourself.' 40 On these two commandments the whole Law hangs, and the Prophets."

How does Christianity define "character"?

This is a too broad of a subject matter to define with any exactitude. So many variables are at play in such definitions and many of them can overlap in individuals!

Modern dictionaries may define it as such:

The particular combination of qualities in a person or place that makes them different from others. - Source

In essence the term character is much more complex than a simple definition.

For example here is just a small example what the Catholic Encyclopedia has to say on this subject matter.

Quite distinct from the technical meaning which the term character possesses in theological controversy is that attached to it in the language of common life, as well as in the literature devoted to psychology, ethics, and education. The interest surrounding the conception of character in these latter branches of speculation has been constantly increasing during the past hundred years.

Types of character

Starting from the basis of the four fundamental temperaments, various classifications of types of character have been adopted by different writers. The intellectual, the emotional, and the volitional or energetic stand for the chief types with A. Bain. M. Pérez, taking for his principle of division the phenomenon of movement, distinguishes characters as lively, slow, ardent, and équilibrés or well-balanced. M. Ribot, proceeding from a more subjective ground of division and excluding indefinite and unstable types as strictly speaking characterless, recognizes as the most general forms: the sensitive, subdivided into the humble contemplative and emotional; the active, subdivided into the great and the mediocre; and the apathetic, subdivided into the purely apathetic or dull; and the calculateurs or intelligent. By combination these again afford new types. M. Fouillée takes sensitive, intellectual, and volitional for his scheme and by cross-combinations and subdivisions works out an equally complex plan. MM. Paulhan, Queyrat, and Fouillée and Malapert have each different divisions of their own, thus establishing, at all events, the impossibility of attaining agreement on the subject.

Ethology

It is very unwise to lay down limits to the progress of knowledge; but it may be affirmed that, at all events, we have at present nothing approximating to a science of character. As we have said, there is already in existence a considerable literature devoted to the psychological analysis of the constituents of the different forms of character, to the study of the general conditions of its growth, and to the classification of types of character. But the results, as yet reached, have little claim to the title of a science. There are moreover two obstacles, which though not, perhaps, absolutely fatal to the possibility of such a science are graver difficulties than Mill realized. Firstly, there is the element of individuality lying at the root of each character and variously determining its growths even in like circumstances, as we see in two children of the same family. The mistaken view as to the original equality and similarity of different minds naturally involved an erroneous under-estimation of this difficulty. Secondly, there is the fact of free-will, denied by Mill. We do not maintain that free-will is irreconcilable with a science whose laws are approximate generalizations as Mill conceived those of ethology to be. All anti-determinists allow enough of uniformity in the influence of motive upon action to satisfy this condition. Still the admission of free-will in the building up of character does indisputably increase the unpredictableness of future conduct and consequently of a science of character.

Education and character

The true aim of education is not merely the cultivation of the intellect but also the formation of moral character. Increased intelligence or physical skill may as easily be employed to the detriment as to the benefit of the community, if not accompanied by improved will. Both do not necessarily go together. As it is the function of ethics to determine the ideal of human character, so it is the business of the theory or science of education to study the processes by which that end may be attained and to estimate the relative efficiency of different educational systems and methods in the prosecution of that end. Finally it is the duty of the art of education to apply the conclusions thus reached to practice and to adapt the available machinery to the realization of the true purpose of education in the formation of the highest type of ideal human character.

Character

St Thomas says "a character means a distinctive sign"

https://www.newadvent.org/summa/4072.htm#article5

He in this article he talks about the "Character of Baptism" and the "Character of Confirmation" being given to people who receive the sacrament. So a Character is a distinction that can be added.

If you're a Christian who believes in the Trinity and Divine Simplicity (i.e. a Catholic), then you don't ascribe "characteristics" to God because that would make Him parts.

He does, however, have attributes. As the mystics may say, "His Greatest Attribute is Mercy". So character doesn't mean attributes.

So, unless I'm using a different meaning of the word character, I think this definition suffices: "something which distinguishes".

Original-sin is a deprivation, not a character.

Although it is proper to each individual, original sin does not have the character of a personal fault in any of Adam's descendants. It is a deprivation of original holiness and justice, but human nature has not been totally corrupted: it is wounded in the natural powers proper to it, subject to ignorance, suffering and the dominion of death, and inclined to sin - an inclination to evil that is called concupiscence". Baptism, by imparting the life of Christ's grace, erases original sin and turns a man back towards God, but the consequences for nature, weakened and inclined to evil, persist in man and summon him to spiritual battle. CCC 405

Baptism is a character, and confirmation aids in the battle against concupiscence. These are marks on the soul of man, which follow us after death.

For us, the living, following interior passions is the disordered way we use our freewill to do evil acts.

Caveat: I cannot comment on the concept of libertarian free will and if character is an essential part of the definition. And I am not qualified to comment on “the rejection of causal determinism when it comes to a person's ability to make decisions”. One thing I do know is that each person is responsible before God for the decisions they make. There are undoubtedly many different points of view within Christianity regarding character, but you may find the following thoughts useful:

You ask, “What is Character?” A.W. Tozer described character as “the excellence of moral beings.” From a Christian perspective, persons of character are noted for their honesty, ethics, and charity. Descriptions such as “man of principle” and “woman of integrity” are assertions of character. A lack of character is moral deficiency, and persons lacking character tend to behave dishonestly, unethically, and uncharitably.

What is Christian Character? The basis of Christianity is Christ Jesus and the basis for a Christian character is founded on the example of the Son of God who came from heaven to dwell with us in human form. The Bible informs us of what God expects of all who claim to follow Christ Jesus. This is what the Bible has to say on the subject:

More than that, we rejoice in our sufferings, knowing that suffering produces endurance, and endurance produces character, and character produces hope, and hope does not put us to shame, because God’s love has been poured into our hearts through the Holy Spirit who has been given to us (Romans 5:3-5).

As Romans 5:3-4 explains, godly character is the result of the Holy Spirit’s work of sanctification. Character in the believer is a consistent manifestation of Jesus in his life. The Lord is pleased when His children grow in character.

You test the heart and are pleased with integrity (1 Chronicles 29:17).

Lord, who may dwell in your sanctuary? Who may live on your holy hill? He whose walk is blameless and who does what is righteous, who speaks the truth from his heart... (Psalm 15:1-2).

King Solomon had much to say about character, especially in chapters 10 and 11. For example:

The man of integrity walks securely, but he who takes crooked paths will be found out (Proverbs 10:9).

The integrity of the upright guides them, but the unfaithful are destroyed by their duplicity (Proverbs 11:3).

Solomon mentions “a wife of noble character” in Proverbs 12:4 then the Epilogue gives a detailed description of the noble characteristics of the woman who fears the Lord.

Other biblical examples of men of character include King David (1 Samuel 13:14) although he sinned on occasion (2 Samuel 11). Then we have King Ahab who may have acted nobly once (1 Kings 22:35), but was still a man of overall bad character (1 Kings 16:33). Several people in the Bible are described as having a noble character: Ruth (Ruth 3:11), Hanani (Nehemiah 7:2), David (Psalm 78:72), and Job (Job 2:3). These individuals’ lives were distinguished by persistent moral virtue.

In my answer above I have partially quoted from this article: https://www.gotquestions.org/Christian-character.html

An important consideration on this subject must begin with the character of God, in whose image we are created. For example, God is love and is wholly good. We can trust Him because He never changes and He can never lie. He is holy, righteous and just. God is compassionate, gracious and merciful. He is also patient and longsuffering. This article expands on the characteristics of God: https://www.gotquestions.org/character-of-God.html

You quote from an answer written by someone else: “Christians have free-will, but their purpose in life is to develop perfect characters that always freely choose not to sin. They can develop this God-like character, but by definition no one can be created with it.”

All I can say is that sinful humans are incapable to reaching perfection through their own efforts. We need help, and that help comes from above as a free and undeserved gift:

God’s love has been poured into our hearts through the Holy Spirit who has been given to us (Romans 5:3-5).

I can't speak for all of Christianity, but here are four examples from denominations that believe building a perfect character is the fundamental purpose of Creation:

Glossary | United Church of God offers these definitions (and others):

Character: The discernment, willingness and determination to make and carry out proper moral, ethical and spiritual choices regardless of the circumstances, pressures and tendency to do otherwise.

Free will, free choice or free moral agency: The freedom to decide without undue pressure whether to embrace or reject a particular course of action; the characteristic of not being governed by prior causes (such as instinct) or divine intervention but able to decide for oneself.

Human nature: The characteristics, tendencies and behavior of a human being. It is fundamentally neutral, although over time self-gratification tends to predominate. As creatures of choice, we are enjoined and expected to resist the pulls of base self-motivation and respond to the leading of the Holy Spirit.

Repentance: A reversal of direction in attitude and actions. We repent when we realize we are headed the wrong way, then stop, turn around and begin moving in the right direction. Spiritually, it involves genuine sorrow for sins and a resolve to do what is right.

Trial: A test of faith, patience or stamina through being subjected to suffering or temptation. We are tried through the difficulties of life. Such tests work patience and build and reveal our character. "If you faint in the day of adversity, your strength is small," wrote Solomon (Proverbs 24:10). Successfully endured trials build holy, righteous character and trust in God.

The Restored Church of God says:

“What is perfect character?” Mr. Armstrong wrote in The Incredible Human Potential. “It is the ability, in a separate entity with free moral agency, to come to the KNOWLEDGE of the right from the wrong—the true from the false—and to choose the right, and possess the WILL to enforce self-discipline to DO the right and resist the wrong.”

Over a lifetime of overcoming sins, faults and weaknesses in the flesh and building God’s divine nature within oneself—through faithful obedience to God by the power of His Spirit, walking in the way of outgoing concern for others and “love [which] is the fulfilling of the law” (Rom. 13:10)—a Christian can qualify—that is, meet God’s standard of approval—to receive salvation and rule under Christ in God’s kingdom.

Does Character Really Matter? | Living Church of God, from yet another organization, discusses the importance of character:

PRODUCING RIGHT CHARACTER

Right character is not simply a matter of knowing what is right; it requires doing what is right! We can identify three primary ingredients necessary to the development of good character. The first is knowledge of what is right and virtuous, the second is habitual practice of what is right and the third is perseverance in doing right—even when facing adversity. It is one thing to practice doing the right thing when it is convenient or even advantageous. It is quite another to do what is right in the midst of stresses and pressures to compromise or to give up. The Apostle Paul explains that "pressure and affliction and hardship produce patient and unswerving endurance. And endurance [fortitude] develops maturity of character—that is approved faith and tried integrity…" (Romans 5:3–4, Amplified Bible).

It is the struggle against adversity that deepens and solidifies our character. If we learn to make right choices even when those choices seem to be to our immediate detriment, we are developing the long-term view that lies at the heart of right character. This view is based upon an understanding of the ultimate reason why right character is so important.

Why Character Matters | Philadelphia Church of God explains the historical importance of character:

Why Religion and Morality? In 1776, the year Thomas Jefferson drafted the Declaration of Independence, John Adams wrote to his cousin, “It is religion and moralityalone, which can establish the principles upon which freedom can securely stand.” There it is again. Why did the Founding Fathers keep pointing back to these fundamental building blocks? Adams himself answered that question in 1798, while serving as president: “Our Constitution was made only for a moral and religious people. It is wholly inadequate to the government of any other.”

They kept referring to religion and morality because, as Adams said, our Constitution was made only for moral and religious people! And why?

Alexis de Tocqueville’s 19th-century observations on the American republic answer this critical question. After touring America for two years in the early 1830s, he returned home to France and wrote his political classic Democracy in America. Like the Founding Fathers, Tocqueville acknowledged that religion and morality were indispensable to the maintenance of the American republic. Why indispensable? He said that while the constitutional law of liberty allowed Americans complete freedom to do as they please, religionprevented them from doing that which is immoral and unjust. In short, Tocqueville surmised, liberty could not be governed apart from religious faith, lest there be anarchy.

Without the moral restrictions of a higher spiritual law, the liberty afforded Americans in the Constitution would be abused. George Washington and the other framers clearly understood that. That’s why they kept harping on religion and morality. They did not want to see the United States of America self-destruct.

The concept of 'character' has developed a certain way with the fairly recent trend that promotes individualism. Perhaps the question has been influenced by this, because I note that in Young's Concordance, the word 'character' appears nowhere in the Bible. Yet it does in some modern versions of the Bible.

For example, in the NLT concordance, it defines 'character' as a noun meaning "moral excellence and firmness; main or essential nature." Then it lists these Bible verses as containing the word 'character' - Romans 5:4; 1 Corinthians 15:33; Hebrews 1:3 (page 2381.) An earlier edition of the NLT uses it additionally in Ruth 3:11; Proverbs 12:4 & 31:10; but not in Hebrews 1:3.

So, where the much older A.V. says "virtuous" in Proverbs, modern versions might have 'good character'. Of note, the NLT sticks to 'virtuous' in Proverbs but has 'character [of God] in Hebrews 1:3.

I make that my introduction to claim that "Christianity" does not define character, but that there is a modern trend in some groups within Christianity to suggest that a high level of virtue is what Christian character entails, even that 'perfect characters that choose not to sin' is what can be achieved, even while in the flesh.

The Bible, I would suggest, takes a sledge-hammer to that notion, because it speaks of our sinful human nature in such a way that our 'essential nature' (NLT) is sinful, or even 'depraved' (i.e. corrupted by our sin), so that we always have freedom of choice (or will) to choose that which is sinful and against God, yet we are incapable of choosing God's perfect good as we stand, in our sinful human nature (or, character.) Augustine had a lot to say about that and his explanations might be a good guide to what "Christianity" has long held to on this matter. Let me quote:

"The core of Augustine's theology was not entirely new with him, of course. Church fathers before him also believed in and taught God' supremacy and the human soul's dependency on grace. But Augustine contributed a new spin to these ideas and linked them together in a new way. As we will see, Augustinianism introduced into the stream of Christian thought something called monergism - the idea and belief that human agency is entirely passive and God's agency is all-determining in both universal history and individual salvation. Many people already know a part of this as 'predestination' and automatically link it with the sixteenth century Protestant Reformer John Calvin. However, the broader perspective is Augustine's monogeristic ideas of providence and salvation in which God is the sole active agent and energy, and humans - both collectively and individually - are tools and instruments of God's grace or wrath. ...While Augustine never completely rejected human freedom, the overall tenor of his thought militates against any genuine freedom of humans to thwart God's perfect will. God always gets his way, even when humans sin and perform evil acts." The Story of Christian Theology, pp.255-6, Roger E. Olson, Apollos, 1999

Augustine interpreted human freedom as the ability only to sin and commit evil apart from God's transforming grace. He elevated God's freedom and sovereignty over against all human freedom and debased human agency as always evil left to its own devices. (Ibid. p.260)

A controversy developed with Pelagius, born in Britain around 350. He wrote a book called On Free Will, to which Augustine responded vigorously. Pelagius claimed that a Christian can be without sin if he or she wishes (if only theoretically). He was declared to be teaching heresy on that and two other counts.

This relates to the question insofar as the issue of free will is involved. My answer is that the modern preoccupation with "character" is a red herring and should be dismissed. Christianity deals with our sinful human nature, and Augustine dealt with that, as it relates to our ability to choose. It is a massive issue, but if you toss out this modern concept of 'character' and replace it with the biblical 'sinful human nature', you should make headway.