Ojou to Banken-kun

Isaku never asked to be the daughter of a yakuza boss, but when her parents died in a car accident when she was 5, her gangster grandfather took her in and raised her as part of the clan. After years of being avoided by her schoolmates because of her family ties, Isaku is finally ready to make her high-school debut, live a normal life, and maybe even find love...until loyal family servant and Isaku's dedicated guardian, 26-year-old Keiya, enters high school right alongside her and vows to protect her from all of the above! Now she's got a chain-smoking, pistol-wielding knight-in-shining(?)-armor to deal with, and navigating high school besides? What's a young lady to do?!

(Source: Kodansha USA)