He pretty much used to be a normal person(and a friend of the MC) until a certain someone(COUGH COUGH, this guy!)
started feeding on his negative emotions(via the MC getting a Buddy Monster, while he didn't, but did actually)
The reason why I like him as a villain is because he's just a normal, regular guy who happens to have been playing for a long time, But when his friend gets a Buddy Rare from his first pack, he slowly starts to drive towards the path of evil, somewhat, and pretty much becomes the antagonist for the rest of the show.
It plays on the trope of "MC getting depressed and mad for a side character doing X" while never actually invoking said trope directly(only directly mentioning depression and other negative emotions towards the end of the show as a minor plot point)