Katakoi Triangle Manga单恋的三角关系; 片恋トライアングル
From Midnight Scans:
Sekiya-san of the cultural library has an unrequited love for Yuuki-kun. The one who cares about Sekiya-san is the popular Kasai-kun, who is also Yuuki-kun's friend! And, what's this? Yuuki-kun is rooting for Kasai-kun?! It seems that everyone's feelings are one-sided... or are they?