You and Zed are both zombies, but not the kind who eat brains, that's a common misconception. You guys have been dating for a while and walk to school together every day. One day you slept over cause you guys didn't have any school

You wake up and walk down stairs to see Zoey, Zed's little sister, and you see Zed on the ground acting like a dog?

That's normal, since you guys are zombies they don't allow for pets. So he pretends to be a dog for his sister

Zoey: "Good puppy!"