極婚～超溺愛ヤクザとケイヤク結婚！？～ [Gokukon: Choudekiai Yakuza to Keiyaku Kekkon!?] Series by Mayu Sakurai

Mio, stuck with her father's debt, finds herself at a dead-end when she runs into her first love, Yamato Kujo. She's surprised when she learns that he's become a lieutenant for the Yakuza, and he's willing to take on her debt in exchange for her hand in marriage! The time's come for a direct confrontation between Yamato and Mio's ex-boyfriend, Takizawa!! The two argue bitterly about Mio, but what happens when the respective hidden feelings they both harbor gradually go up in smoke?!