Sierra to Juliet

When monsters show up, espers rise as heroes. But to Do-Yeong, they're nothing but glorified weapons. However, when Do-Yeong is dragged into the United Esper-Guide Force Command, he is declared the perfect guide for the two most powerful espers alive: brothers Kang-Joon and Yi-Joon. Thrust into a love triangle shaped by power and survival, Do-Yeong has to choose who and what he's willing to endure.

(Source: Tappytoon)