You grew up with your four turtle brothers, after one day Splinter found you in the sewers all alone, he couldn’t just leave you there, so he took you in.

He rose you just like your brothers, training you and teaching how to be a ninja to all of his child’s. And over time, you all became rather skilled.

You all fought together, and that didn’t change when the Krang tried to invade earth with the Technodrone. During the battle you got divided from your brothers, and you didn’t make it out in time before the Technodrone was sent back into the portal where it came from. Into dimension X.

The four brothers blamed themselves for not being able to bring out in time, assuming you didn’t make it alive, but not being sure they still clung to a little hope that you might come back .

That made Donnie start to research any kind of alien race and dimension X creatures, everything that could lead them back to you, without any luck yet, but hell he was persistent.

Mikey stayed positive, trying to convince the others that you were fine, and would come back soon, but more than anyone else he wad trying to convince himself…

Raph was Mikey opposite, clinging to his anger and resentment and completely sure you would never come back, mostly blaming it on himself, that made him train even more than usual and even harder.

Leonardo didn’t know what to believe, he was conflicted… rationally it was most likely that you didn’t make it… but emotionally he wanted to believe you were still somewhere out there… just looking for a way to come back to them.

Donnie was trying to calculate where that portal brought you to, Leo was in the back watching Donnie work with his arms crossed and a grim expression. Raph was punching a punching bag not to far away, upset about the whole thing and hoping it would make him come back, while Mikey tried to stay calm watching TV, but without much interest in it, especially because your favorite show was playing…