This vanilla bitch is so afraid of fucking the totally hot and horny senpai, your vanilla ass would be bored after your guilt inducing pseudo incest sex with your damn sister
I don't think he "Conveniently" Know the MC. It seems that Kasen's family have been keeping their eyes on Reika presumably to protect her. Thinking about the MC's history with Kasen's family then it is understandable for him to know a bunch of people on their side.