I don't think it Ntr or anything like that. but I do believe back in middle school she liked to bully and tease him, resulting in him becoming the isolated loser he is now.
@admmi I believe he stated it earlier, but it's not like he is a girl now because he likes guys but because he really likes Yo. But Yo prefers a normal boy-girl relationship so Kei decided he would be that "girl" for Yo.