You where a normal teenage girl until you ran it the wrong people. Your body got mutated and you became a mermaid withe the power to control water. The turtles start to take a liking to you as well and splinter treats you as on of his children.

I DO NOT OWN TMNT!! Y/n is a street dancer that's trying to make a name for herself. She moved to New York hoping to be discovered or to learn as many new moves as she could. What she ended up learning was that turtles are pretty good dancers. Also having a tail is really useful.