I, Kotaro Hasegawa, have what you call "Onee-san Complex".

During my high school, I confessed to a Senpai during my high school years and was rejected.

Years passed and I'm now at college taking up a teaching degree.

During my college years, I encountered a high school girl being picked up by strange guys and helped her.

As part of the teaching course, I was assigned to Fukui High, where I graduated and met yet again Senpai.