Foster's Home for Imaginary Friends. He serves as the main character/antagonist throughout the series. He is the only one to appear in every episode. He is known as Mac's best imaginary friend, who Mac adopted in Foster's Home, a place where other imaginary friends are and they can get adopted anytime but the other imaginary friends that's there we're either given up or has not been adopted yet. Bloo was later given up by Mac at the end of the series but still gets visits.