What about the dancing games? Those are developed in house by Atlus and use Ren Amamiya/Makoto Yuuki? For me a name is "canon"ized when they use it in the main game (P2) or a canon sequel (P4AU/P4DAN). The P3 and P5 dancing games aren't considered canon, but they're as close as you can get imo.

What do you think of Tatsuya Suou and Maya Amano? They're renamable but default to those names when you don't change it plus that's what they're called in the game they're not the protagonist in. What about Yu Narukami? The game doesn't default to that if you don't input anything plus he has a different name in the manga but I still consider Yu canon, especialy since P4DAN is kinda referenced in P5.