In "Supplies Party", Chad's bullying of Adam went way back to when he was 6 years old. At Adam's birthday party, he wet his pants and Chad laughed at him in front of everybody. Adam never lived that down and Chad still reminds him of that incident today. 6 years later, Chad awaited Adam's next birthday party, so he could make fun of him once again. Since Adam thought Chad was a friend, he invited him over. At first, Chad appreciated the party that went on, but eventually, water spilled on Adam's pants, emulating the wetting he had at his 6th birthday party. Chad laughed in his face and Adam cried the same way he did when he was a child.
In "The Times, They Are Exchanging", Adam went back to his old school, and met up with Chad again. Chad was in the process of throwing a kid in the trash can, when Adam came up to him, and got himself insulted. Adam apparently thought that Chad was his friend, despite the obvious mockery that was given to him. Another moment at the new school involved Jake jumping up onto Chad's head and monkeying around with him. Chad started freaking out and calling the monkey a "freak" and started slapping it and eventually threw him across the halls. Chad got in no trouble for anything he did.
In "Lonely Lyon", Chad was getting bored of bullying kids at Chester Arthur Middle School, because none of them gave him the same thrill as Adam Lyon. He put on an elaborate scheme to get Adam back into his life. He then kidnapped the CDMS student Bear Ted, dressed up as him, and pretended to be him, just so he could get into the school. Chad also wrote a false letter to the newspaper, under Adam's name, saying that he wished his school treated him more like an animal, which made for Adam's life going to an all-out hell. After Chad was busted, he was banned from the school premises, making for quite possibly, the only time in the series, Chad had ever gotten his comeuppance for what he did. This is also notable for being his final appearance in the series, basically marking the end of his story arc.
Chad is a generic school bully, who makes fun of those who are different than him. He mainly targets Adam, since he goes to an animal school, making him just about as different as it can get. Chad is rude, abrasive, violent, and insulting. He's also not too bright, and he knows it. His go-to insult is to call people "Stupid", a word which reflects perfectly off of how he sees himself. Despite being a complete bully, he's on the good side of many, if not, all of the students at his school, including Kerry Anderson, a smart girl, who should know better than to hang out with him. In fact, even Adam, himself, treats him like a friend sometimes. Adam is full-aware of Chad being a bully, but for some reason, allows him into his life sometimes, possibly because of their "friendship" dating as far back as to when they were 6 years old. Chad is a real single-target bully, as seen in "Lonely Lyon", where Chad got bored of bullying the students at his own school and went out of his way to get back int Adam's life, just so he could enjoy the thrill of bullying him again. Chad's elaborate scheme had to have been the smartest thing he's ever done.
Grim Adventures of Billy & Mandy