The Grim Adventures of Billy and Mandy (2001)

Season 3: Episode 21

After being bullied again by Sperg, Billy asks Grim to shrink him to enter Mandy's head to steal her "nerve". After Billy swallows the raw nerve, a polyp-like organism, Mandy becomes timid and scared of everything while Billy becomes tough and mean. While Billy goes to get revenge at Sperg, Mandy enters Billy's head to recover her nerve. Mandy recovers her tough and cynical attitude without her nerve, which is successfully retrieved from Billy and banished from Endsville.