The side of Yukihira-kun I don’t know. My younger co-worker’s devotion is too heavy.

Mia, a office worker is dating someone who's older than him, Mizuki, but she is treated like a trophy as
a "younger girlfriend" and pretends not to notice it, even though she is treated sloppily. While her bf was  busy with work, she was saved by the kindness of Yukihira, a junior at the company, but when she suddenly complained about his boyfriend … Yukihira's appearance changed completely.