In case you ever go back: https://www.bmobile.ne.jp/english/
I've used this on several trips and it works well.
Unless of course you were living there more permanently, in which case the above is not useful.
With my current prepaid SIM, I typically top up $5 every 3 months. Between iMessage, WhatsApp/WeChat/KakaoTalk/Skype/Facebook, free incoming calls, and the occasional emergency SMS, I usually don't need a phone. People don't like it when I don't have a local number though.
That's cheaper than most plans in any country, let alone roaming.
Yes, I pay per minute for outgoing calls, or MB of usage - and those costs are high. That's why I try to use WiFi whenever possible, and use offline maps, offline Wikipedia, etc.