Tmnt Little Sister

So, yes. You read correct.
(This is more of an OC book btw.)
Mostly, the Story of how Riuki Lyn Onodara, a demon girl, met the Teenage Mutant Ninja Turtles and became their sister.

彼らが立っているユナイテッド、彼らが落ちる分割。ハウスハマト。いつもそして永遠に。

Translation:
"United they stand, Divided they fall. House Hamato. Always and Forever."

(If you know where this saying is from, don't come for me in the comments! I know what I'm doing!)

Follow her through her new life. Finding love, happiness, friendships, anger, her powers, and some betrayal thrown in.