Koisuru Gokudou Oniisan

Akihiro Kashima is the right-hand man of the Sawatari mob, and he has a secret. He's been hopelessly in love for years—with his boss! As a trusted subordinate, he's subject to the head honcho's unintended acts of seduction. But with Akihiro's position in the group at stake, he couldn't possibly reveal the infatuation that plagues him. Stuck between duty and his feelings, something's gotta give!

(Source: Square Enix)