Took me a while to think and see what is the one trait that branches across all my characters, but I think it might be that I never really sit on one character but usually have a pair of sorts.

This can vary in different ways and probably why it took me to notice. Some will have dual personalities, some may be siblings, two rivals, and so forth. Basically it'll usually turn into a situation where the whole story will be a construct of the two characters and each has a link to another.

One of my favorite examples would be my character Eppie where she was able to mutate into a lovecraftian creature cause she had a parasite monster that lived inside her.