You were shredder's kid. You had run away a long time ago at a young age. Splinter just so happened to find you and took you in. You were raised alongside the turtles, they were like brothers to you. You would train with them and everything. Splinter was the only one who knew about the truth. One day, the turtles were planing to attack shredder's hidout. But the one problem was, you could go back there. There were too many memories there that were painful and you just couldn't relive them and see your father.

Leonardo- "So were gonna need Raph and (user) to attack from this point." he said as he explained the mission like it was a regular one