I've never had a mentor. I've always really wanted one. I have, like many people, got snippets of advise from professionals at conventions but that's really it. It always bothered me as I was learning that no one wanted to give me the time of day to help me. So, now that I've learned a lot (usually the hard way, by failing) and I've published and feel pretty good about what I have created, I try really hard to help other artists. So I've done mentoring for a number of comics.
I do however have a mentor for marker and commission work. She has done work for Copic and has a bunch of how to draw manga books out. She really got me started and keeps me supplied with Markers. She taught me so much and is all around awesome.
Here is her page: