It wasn't Sakae's father. It was another student's "guardian". It's not even said whether Ryuuji was being stalked by a male or female (could've been either since he is very handsome).
You're totally right, I re-read to be sure. I thought I saw a drawing of a man like a memory but no, my mistake. Thank you!
I assumed the stalker was a female because it looks like Ryuuji had gotten into trouble for it (even though it wasn't his fault). They transferred him to a different school following the incident and he purposely made himself less attractive after that.
That would have been screwed up if it was Sakae's dad! I suppose there could be a slight possibility, but it was sometime after Ryuuji taught Sakae that he was stalked. When Sakae was a young teen and saw him again, Ryuuji was still clean-cut and Sakae's mother had remarried by then. Anyway, I think there's a good chance Ryuuji would kicked that man's ass knowing that he had abused Sakae and his mother.
AAH! My mistake! Sakae's mother was divorced and not remarried yet.
I wanted to know who the parent/stalker was. Was he Sakae's dad? I wanted to know about Sakae's past and Ryuuji's aswell. This manga could've been longer and better if the mangaka invested in it.