Upvote:1
Rentals made sense back when Japanese phone networks were largely incompatible with the rest of the world, but those days are long gone. Free WiFi is now available in some places, but you usually can't rely on it being available when you most need it, and there are often annoying registration hoops that either require understanding Japanese, having a local number or both.
Your best bet is to buy a local SIM card, and you can do this easily and cost-effectively right at the airport on arrival:
Where to get data-only SIM in Japan
Prepaid SIM with voice & data at Haneda Airport
Note that voice & data SIMs are more expensive and limited than data-only SIMs, so pick accordingly.