Hugo Simpson II is Bart's conjoined twin. Shortly after their birth, they were separated by Dr. Hibbert.
Hugo and Bart were conjoined twins, born with a band of skin connecting them side-to-side. Dr. Hibbert conducted a "soul smear" test which revealed the presence of pure evil in the left twin. He then separated the twins with a paper cutter and sent Bart home to live with the family. Hugo, however, as the "evil twin", was consigned to living in the attic of the Simpson home and once a week was given a bucket of fish heads to eat. Marge and Homer would make mysterious references to these feeding times, with Marge saying things like "Isn't it about time for the ... you know?" and Homer replying, "Yeah, yeah, I'll go feed it" and carrying the bucket of fish heads to the attic. Bart, Lisa, and Maggie discovered Hugo after they heard strange sounds coming from the attic and after Homer spilled the beans about there being four Simpson children. Their curiosity piqued, the kids went exploring in the and found Hugo.
Hugo said that he had been doing experimental surgery to practice for eventually reattaching himself to Bart, creating a "Pigeon-Rat" in the process. His plan nearly succeeded when he pretended to escape from the house and everyone.