Zero is the antagonist of Millard's Home. He is ab anthropomorphic cat like demon. His backstory is unknown to everyone but he just knows that he hates Millard. He actively tries to get inside the mansions pocket dimension but has succeeded until now. 

Zero has a deep voice that is very raspy and creepy. Zero is sadistic as hell and loves to toy with the emotions of Millard by threatening to take over the mansion. He isn't seen very much but can be heard as a voice in Lester's head due to him having possessed Lester. 

Spoilers: Zero is Millard's ex husband whom got overtaken with demons one day. Zero was once an angel if great beauty along with Millard but got corrupted. All his good memories of Millard are replaced by bad ones so he thinks of Millard as his enemy. Millard knows that Zero is his husband but Zero doesn't remember properly.