I think the popular theory is that Suo used to be weak and that's why he has such a strong conviction in helping/protecting Nirei and while that theory makes a lot of sense there's also a lot of evidence of the opposite being true

The most consistent thing Suo turns to when insulting an opponent is calling them "childish", and he does this in the same way/situations in which Sakura would use "weak" and then he'll 'teach them how to grow up' so my theory is that Suo was someone who went after weak people himself and has learnt better since and sworn to protect the weak instead this both explains his immediate protectiveness of Nirei and his repeated ideal of "maturing" into having empathy

also while we're here I just want to throw out there that I highly doubt Suo's master is an adult, I'm almost certain it's an older kid that was also a delinquent just based of the fact that

1)there are no plot relevant adults in wind breaker idk why we'd start now and

Sethos- after the funeral for his grandfather

I honestly feel like these are really important lines and I'm surprised they are locked behind Nahida

this was just funny to me. some haikavehass behaviour tbh (except they were like so chill in this quest they didn't even argue once)