Never hurts to ask, as Michael said! But here's what you can and cannot do with a "Yes" reply!
You can:
Feature their character in a non-major roll for a very short time (cameo as it's known)
You cannot:
Make revenue off of your work. Unless the artist and you sign a contract (say if you go to premium or make money off of your creation) you need to either pay the cameo'd artist a lump sum for use of their character for X amount of pages, or a cut of book sales, or no payment at all. But you NEED to have a written and referenceable document stating the terms otherwise you're totally messed over if you ever make money and an artist you asked pops up and (Not saying they will) lie and say you did it with no confirmation.
I've seen great comics go down because of small things like this! Be careful!