Risa is Junichi's secret stalker.

Spoiler

Her extra ending in the game. Risa Ends: Risa is your secret stalker. She prevents the famous rejection many years ago since that girl was doing it for kicks. Her path is also activated when you activate another girl's path. Risa will sabotage it by spreading lies and making the girl hate you (Ayatsuji will be the only girl who won't believe her). - If you accept Risa, you'll be a happy couple in college. - If you refuse her, she will tell the teacher's about your secret porn stash in school and you will be suspended. Rumors that you're a pervert will circulate around the school. source: http://pastebin.com/T97YkmaQ