Tetsuo is worrying over nothing lol. Maybe it's true that Kasen idolised Tetsuo for a long time, but she's gone to meet with many people in college since and yet still connects with him the most. Whatever idealised version of Tetsuo that Kasen envisioned all those years would've shattered the more time spent she got to know him now, but that doesn't seem to be the case. Her feelings are definitely genuine.