When Characters Pop In My Head

So...I'm supposed to be working on finishing up the partial my agent submitted a few weeks ago. It's a story that I really like, with characters that I can see having fun and growing. It's the first of a series, so I feel like there are a lot of opportunities.

But I've got this nagging character from another story chirping in my ear. I'm pretty sure she's the same one that's hanging around in a women's fiction manuscript I started, but haven't finished. She's been bothering me big time lately. So much so that I sat down and wrote a page for her. She's quieted now for the moment, but I know she'll be back.

They always come back.

I know this sounds like I'm crazy. But if you're a writer, I'm sure this doesn't sound any different than what goes on in your head - at least I hope it doesn't sound any different! If it is, someone call the white-jacketed men so they can take me away!!

What do you do to calm those pesky characters who want to be heard now, who don't want to wait? Do you write a bit? Do you outline their story? Do you take notes and then get back to your current wip? Or do you ignore them all together?

I know this chick is not going to leave. I've been thinking about her story off and on, knowing that I needed to return to it at some point. I've put about 25k down in words for that story, and I don't want the time I spent on it to go to waste.

What do I tell her? I've given her a new beginning, and she's happy for now. But that won't last long. Sooner or later, I know I'll cave and get back to her, even though I've told my agent that I want to write MG now. Guess I can't help my women's fiction roots.