i never talked about this openly bc i feel like it could be misunderstood easily but essentially he's transmasc through being brought up as a boy (he's actually agender so like still not c1s but not a trans man either). i like the idea of yuzuru's "his life was never his own" canon extending to his gender expression as well. as for why sth like this happened, since yuzuru's parents suck and only see him as an object to accompany tori, when tori was born and got assigned male, they just started raising infant/toddler yuzuru as a boy so that the 2 of them would never get separated bc of gender segregation issues. i really like reading this headcanon into yuzuru's half-hearted disapproval of being called by feminine terms like "mother" or when mika calls him "not really a man /pos": it's not that he's emotionally opposed to not being seen as a man but he knows it goes against his assigned "role" so he mildly corrects people
also i hc that ibara will come out as transfem later in life and that will be the catalyst for yuzuru questioning himself and figuring out he doesn't really feel a particular gender ^w^