Yes, I said the center of the character.
I'm sure it sounds weird, but it'll make sense in a moment.
A few years ago I finally got to take the class I'd been waiting for. Creative Writing. I was stoked. Being a business major, you have to pick up the arts when you can.
Boy was I off on my excitement. The teacher has a highly narrow view of how to write, and is one of those 'follow all these rules, and you'll be good.' only to correct you if you don't follow them. All this added up to me having 0 creativity in the class, and for a month or so afterward.
I did take one thing away from it. He told us that a good character can be pulled from any world and tossed into another, and survive.
At the time, because I wasn't enjoying the class, I thought he was an idiot. Then a friend and I wanted to take our characters [Ezio and Mal] and use them in another world. Some random place we'd make up.
Turns out, he was right about that.
I took Ezio out of a modern world, as an outlaw biker, and stuck him on another planet. But he's still Ezio. Yes, there are differences but what I call the center of who he is, is the same.
What makes him... Ezio?
He's an artist. Kid is freaking good at it too. He can paint, draw, sculpt, and do other forms as well. Art is in his blood.
He can't cook. Blows up kitchens. Not sure how, but he does. Unless is French Toast, have 911 on hold if you let him. [which is stupid, because he'll blow up your kitchen.]
He's good at reading people. This works its way out in different ways. Sometimes he's a seer, sometimes he's just good at it, sometimes its hardly around. Unless someones interested in him, that he can't notice.
He's a fighter. This is harder, since I tend to put him in the role of Traveler or Roma [Gypsy] a fair amount. Often, he's been kicked out because he'll fight.
He's gay. Yep... Always. His exact words from having normally slept with one woman in his life, is that girls are squishy.
Those are the keys to Ezio. No matter what world I plop him into, those things pop up. In some worlds he's more broken, others he's more promiscuous, but those are the key to him and always show up.
Its a good idea to learn the heart of your characters. What key elements do they HAVE to have to be them, and what could they do without?
You can also do it with things like... Druids, Fairies, Myths and the like. If you're gonna change them some, research the various truths, myths, and legends of that theme and find what's always there. Then you can change them some, but always keep those key elements in place.
Hope this has helped. Remember, its only my thoughts on the whole thing, not a rule or a have too.