Boku To Mimoza No 75-Nichi summary:

Rindou Shuuto is the son of Yakuza boss Rindou Takeshi, the brutal and voracious head of the Rindou-gumi. Takeshi is likewise a heartless and abusive parent, very much angered by his son's timid nature, but Shuuto finds comfort in the companionship of his new bodyguard, the beautiful and mysterious Mimosa; whom he dubs as his "older sister". Later though, Shuuto walks in on his father apparently forcing himself onto Mimosa. Reaching his limit, Shuuto violently attacks his father with a knife, but it's Mimosa who swiftly finishes off the crime lord, protecting the young man. The two proceed to go on the run together....