Citizenship is as easy, immigration is a bit more complicated......
https://www.juridique.jp/immigration.php
As for my own situation, I was on a Humanities visa for 5 years, then an Instructor visa for 5. Got a Permanent Resident visa last year which makes as close as you can get to being a Japanese citizen without handing in my soon to be useless British passport. Getting citizenship would be a piece of piss for me.
I'm enjoying watching you get schooled by people much more knowledgeable than you on this topic though....Your attempt to save face by implying that I might not be accepted on the grounds of poor mental health or a criminal record was particularly amusing.
To be fair, your username does suggest that you might be a bit .... well .... mental !