Kanojo no Tomodachi

Takeru Hidaka is an ambitious high school student aiming for medical school. He has just started dating his first girlfriend, Kaori Koga, and to everyone else, their relationship seems perfectly harmonious.

However, that peace is threatened when Kaori's best friend, Tomoko Yoshioka, begins to flirt with and seduce Takeru. Caught between loyalty to his girlfriend and Tomoko's advances, Takeru falls into a double life and a complicated affair. Now, he must navigate a web of lies that puts both his academic future and his emotional connections at risk, all while the constant threat of his secret being exposed looms over him.

(Source: MAL News)