Well.. I'm abit disappointed that the author copped out and went for the easier route in solving the love interest problem (by simply giving the senpai an obvious bad personality). I was hoping that the senpai genuinely cared about her and just went overboard with his attitude from his initial surprise of the news. Before this development, his reaction was normal. And given the ossan's explanation here, it justifies why the situation was sketchy. Any normal person should be worried if someone they liked got involved with some random dude like that.
Also, with how fast the FL's love interest switched to the ossan, I'm slightly confused as to why the senpai was even necessary in the first place. Her crush for him seems superficial now. I thought the story had something to do with her struggle in choosing between fulfilling a long unrequited love and a new flame. So with this development, I don't see a point in his character in the first place. xD It makes the FL look slightly bad too, cause it gives the impression she becomes infatuated easily.
Dude I am thinking too much x'D