The Side of Yukihira-Kun I Don't Know - My Younger Co-worker's Devotion Is Too Heavy

Mia, an office worker, is dating Mizuki, a man who's older than her.

But he treats her like a trophy as a "younger girlfriend" and she pretends not to notice it, even though he treats her sloppily. While her boyfriend was busy with work, she was saved by the kindness of Yukihira, her junior at the company. However, when she suddenly complained about her boyfriend... Yukihira's appearance changed completely.

(Source: DLsite garumani)