So the oldman was this author's self insert, now the MH has a older sister we didn't know she had and of course easily goes for the oldman "to protect her sis" BS.
Karin wrongly thinks her sister is being coerced, and agrees to spend a night with the teacher to end a nonexistent blackmail. As a nonvirgin who never comes from sex, she's not fazed at all. Then he destroys her in bed.
https://e-hentai.org/s/20fa538524/3298875-90 In middle school, Karin dated a stud from the basketball team, but he always finished fast as she was hotter than his past girls. She didn't like sex, and usually opted to give him a blowjob. Before college, they broke up, but remain friends.
Lives as a popular makeup streamer. Masturbates thrice a week.