The bad guy. The villain. The one who cheats, talks trash, and makes the crowd want to throw things at the screen. Great heels don't just get booed, they make you genuinely angry. The best part? The loudest boos usually mean the biggest merch sales. See: MJF, vintage Triple H, literally anyone who has ever hit someone with a chair during a handshake.
"He turned heel at WrestleMania by attacking his own tag partner. The crowd lost their minds."