For a very long time I've had three semi fluid character designs, and I had never really bothered to differentiate them in any way. We had Shredder, Kai and Krysch, three parallel reality versions of each other.

Now however, they officially have their own designs,
Krysch, a darker soul, bonded to a godlike creature. He is immortal and possesses powers over the fabric of all creation. He is very direct and can be quite callous, caring little for the consequences his actions might have on others and only for doing whatever he sees as necessary to protect his own reality. He hasnt hesitated to wipe entire worlds from existence.

Kai, a pretty innocent character who just wants to make people happy. He is pretty smart and whilst he doesnt have any powers or great strength he can figure his way out of a scrape. He is the smallest of the three but his energy and heart are probably the biggest.

Shredder, (AKA, Peter 'Shredder' Weber). This muscly guy has gone to great lengths to establish himself as someone in charge who gets whatever he wants. He is very much a bully, and takes great pleasure in it, though for the few people he does care about his heart runs very deep. In an encounter with Krysch he gained the ability to phase short distances and this also allows him to create temporary copies of himself by phasing rapidly between different points in physical space.