Taiga's dad

I see some confusion in the comments; yes, he was a cheater, but on the day they were visiting the hospital for the last time and the dad saved Arima, he got someone to call the police and the teacher got arrested (it's mentioned he was on parole when Arima collapsed in the play).
The dad had bought the flowers for the mom, he mentioned this himself, and he stayed with Arima until the police came instead of going, since "my wife wouldn't wish for me to abandon a girl in need". He wanted to apologize and "redeem" himself, but was probably nervous and needed to walk around a bit, hence why he was around at some random bathroom, not with another woman or anything.

So though he is indeed a shitty person in terms of being a cheater previously, the rest of what you all are saying are not true.

Anyways, love this story and hope the author is resting well.