This is maybe not a popular opinion but I feel like the kouhai doesn't have any feelings towards her, this is a generic scenario that I had in mind:
S= Senpai
K= Kouhai
S: *Saying something Tsundere-ish, then somewhat confessing
K: *Surprised, smile then "Thank you S, but-" proceed to explain that he had significant others
S= *Cries
Pls, don't bully me XD