A Throne of Fire

Tsunami Sano was four when she met a boy wielding tonfas and a man in red. She was four when she felt the warmth of her flames for the first time, and she was four when a man with short blonde hair in a suit tried to take that warmth away.

She was barely a week older when she got it back, and Manjiro Sano was not at all happy with Iemitsu Sawada.

Or, in which Tsunami Sano is the only child of Manjiro 'Mikey' Sano and Nana Sano nee Sawada, who unlocks her Sky flames at a young age, bonds with the Storm Arcobaleno and his young great-nephew, and later becomes the heiress to the world's most influential mafia familgia, picking up an army of loyal guardians and a personal male harem along the way.

Nana is very amused. Mikey is not.