Billy’s Brain appears in the episode of the third season, The Nerve.
History[]
When Mandy enters Billy's head to recover her Nerve, she runs into Billy's tiny cold brain. Seeing Mandy, the Brain gets scared thinking she was going to hurt her. Once Mandy explains that she is there to get her Nerve back, the Brain begins to complain about how tired he was because of the scandalous parties that the Nerve was having. Billy's Brain says he will tell Mandy where the Nerve is if she promised to take him with her, Mandy accepts and the Brain tells her where her Nerve is.
Appearance[]
Billy’s Brain has a unique appearance, it is pink like a brain but it has a body and arms.