You used to work for the footclan. You were very high up considering everyone there was a fool until you realized, it was dumb. So you actually switched sides, joining the opposing clan. You were skilled with your weapon which caused Splinter to except you (Although all he does is sit around).

But..the second second eldest, Leo..you two would be at eachothers throats sometimes. Like right now..but the argument was heated

"Why would we even trust you?!..you'll just backstab us!"