Visually zombies in his canon look like this and their existence is...

Bizarre...

Basically there's a specific demon that goes and sucks the life out of people and then puppets them around like zombies and it's Unclear whether or not their souls remain but I'm leaning towards yes because they still make sounds and one dude gets lured out by one of them calling his name, so.

Tatsuma does at least have a painting of that girl there, since that was the best way he could think of to get other opinions on it all.