The kept in "kept girl" implies two things: One being that they are well kept for financially, the other being that they are kept by what could be considered their owner, ie. they are "keeping" her.

"She's a kept girl."

She has had a very self-kept life due to her parents strict rules and guidelines.

This car has been garage kept. The body's excellent.

“Damn girl, u lookin all kept since you got wit him...”

"Hey did you hear that new song?" "Yeah bro I'm kept"

"Hey the part will start at ten ok?" "Sure man, I'm kept"

She used to be independent now she’s a kept woman.

You are a kept nigga!