Can you have too much character development?

I'm working on a grimdark dark fantasy and thought I'd include a small character on the side that has a dark twist to it. This is a fantasy world so I figured I'd play with a character who's backstory is a pretty common fantasy cliche, but with an unflattering dark twist. The character herself is a former slave who escaped and started to work as a mercenary. The thing is that she escaped from being a slave because of a noble's sympathy, and he used her as his fucktoy, then when she asked for actual real freedom, he realised he didn't actually care about her and he actually just wanted to fuck a slave. So she ended up killing him and fleeing. She's not a hero and doesn't actually do heroic actions, and she's not going to end up in a happy situation like other characters in the story. <br><br>But the more I write about her, the more I start to feel like I'm actually ruining the main storyline with too many subplots and side characters. The story's got other characters, other plotlines, and a fair amount of grim darkness. The more I add to this character, the more I actually feel like I'm detracting from the story overall, especially as I'm realised that this character isn't actually going to actually achieve much in the story and is just kind of there as a side character. She's got friends, she's got motivations, and more and more backstory, and the more I write about her the more I feel like I'm overdoing it and actually detracting from the story. Can you have too much character development? How do I know whether I'm going too far with this?