It's like... art style, theme, and branding all rolled into one, I think?
Like my friend has a very solid aesthetic, to the point her original stuff vastly outsells her fanart.
I'm sure you can see why. Her sense of composition is phenomenal and the ideas she puts on paper are always creative and unique.
You could call her stuff:
She appeals to a very specific audience and does a pretty good job tapping into it. Don't expect to find yours quickly--but think about your art and what you like to convey in it. Why do you draw what you do? What emotions or feeling are you trying to get across?
It doesn't have to be super deep--some people just make their aesthetic cute shit or epic fight scenes because that's what they like doing. But try to think about what you like to say with your art and do more of that. Genuine stuff will always sell better than something forced, anyway. People can tell.