Though they are undead, they're not ghost. What you don't realize is Pac Man can only eat ghost, and that's when he's in "power-pellet" mode. Even if he could devour Alucard, he's way too fast for Pac Man. He'd just keep back stepping, and then stap Pac Man in the head when he ran out of juice.

I was thinking more on console video game characters because I don't play computer games for shit.