M is R's best friend, as far as zombies have friends. He's described as "hundreds of pounds of muscle and fat draped on a six-foot-five frame. Bearded, bald, bruised, and rotten" (1.1.11). If we take the Romeo and Juliet theme running through Warm Bodies and run with it, M is a pretty appropriate first letter for R's best friend. What was Romeo's best friend's name again?
Like Romeo's best bud, whom we describe as "dirty and funny and out of control," M is all of the above. He lives in the women's restroom, loves porn, and is "the only zombie [R has] met who's managed to maintain a dangling scrap of humor" (1.8.101). We imagine clinging to your laughter in the zombie apocalypse is quite the feat.
Unlike Romeo's best bud, M doesn't die. Well, he's already dead, but you get the point. Another difference between M and Shakespeare's classic comic relief is that M is totally into the idea of love. It may or may not be mostly physical for him, but he desperately wants to feel the sensation. Right alongside R, M is one of the first zombies to start going through the change (it's like menopause for zombies in reverse) and getting in touch again with his Living feelings.
He even ends up hitting on Nora at the end of the book. She must have a thing for tall, gray, and husky because she seems to totally dig it. He tells her is name is Marcus. That could totally be a nickname for Mercutio if you have the right accent. Or not.