When I read the Japanese ver, it doesn't really feel like there was a romance system. It seems that during the development stage, a feature was planned where you could interact with students like a companion mod, so it's more of an evaluation based on frequency.
Hmm, not really. I have access to the game files, and I can tell you that the NPCs make remarks like that about all of your companions (including cut ones like Imelda and Everett), not just Sebastian.