I’d probably say nano from the 2003 show, in every episode it appears in, I can’t help but almost shed a tear, especially during the first two seasons.
(dramatic comment aside) it might’ve been created to be a robot, or even a weapon of some kind, but you soon realize that it’s just a kid who just wants a family. And sees it’s creator, and the crook as its parents.
I like the premise of nano because its want for a family contrasts between the turtles. And looking back at the episodes now, it makes me wonder, what if splinter never found the turtles pre-mutation? They were just babies after all, struggling to figure out the world.
When they eventually stop nano, they take a time to reflect, to appreciate that they have each other, splinter, even their new friends April and Casey, There’s a chance that the turtles probably could’ve ended up like nano without the guidance of their father. Their family.