Obviously a demon to anyone who has even a little bit of knowledge about possessions and how they work.
Chris Chan fulfills both the requirements for becoming fully possessed, being both mentally weak and having been debilitated, the demon probably attached itself to Chris around the time he was being abused by his babysitter or when he was restrained at school, it would then oppress him for a while until it became powerful enough to manifest itself as a combination of the two things Chris love the most. Demons take whatever form they think will be most useful in getting what they want, for most people, this could be the spirit of a loved one, Christ, the devil, God, or some other deity, for Chris however, it would be a fusion of Sonic and Pikachu. He also apparently hosts multiple demons, which is common, Magichan in particular seems to have a lot of control over him.
The demons have the ability to assume direct control of his body, which is rare, and a sign of grave possession.
Chris was just a regular weird guy until highschool, when Sonichu first appeared, it consumed him and became the most important thing in his life, he went through the stages, demonic influencing, obsession and then full possession in order. It's also possible that he was initially possessed by Bionic, who was a weaker demon that invited the more powerful demons that would possess Chris later. It's not impossible that Sonichu and Magichan are real shit powerful demons that you shouldn't fuck with.