The Grim Adventures of Billy & Mandy

Whatever Happened to Billy Whatsisname? (3x28)

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When Billy and Mandy have a brief fight, Billy says he never wants to speak to Mandy again. The next day, Billy is over his grudge, but Mandy has already decided to find a replacement for him. Later, Mandy introduces Bobby, the new "Billy", who turns out to be a better friend than Billy, even gaining the affection of his parents, but who also wants to take over Billy's life. In the end, a jealous Billy uses Grim's scythe to open a portal and send Bobby to another dimension.