When Jeff decides they need a child in their lives in "Season's Beatings", he and Hayley adopt a son from an orphanage. Stan Smith discovers he bears the numerals 666 which is considered the mark of the beast and is convinced the child is the anti-Christ. Stan gets Father Donovan to come see the marking and they agree to go to the orphanage where Nemo came from only to find it burned to the ground. as they search the remains of the orphanage, Father Donovan discovers Nemo's birth papers. Nemo senses the Father's discovery and causes the orphanage to collapse on him, severing his legs. Father Donovan is able to share that Nemo is indeed the son of Satan before he dies.
Stan, Roger and Jeff travel to the Vatican to obtain a set of sacred daggers to kill the anti-Christ. Although Stan is barred from entering, Roger and Jeff are able to enter the catacombs to search. Nemo sees them through the eyes of a rat and causes the catacombs to flood, washing Jeff away. Roger climbs out and finds the daggers and escapes with them after attacking a Cardinal. Roger and Stan return home to discover Francine putting up Christmas lights while a possessed Steve scampers around. When Stan tells Hayley that they need to perform a ritual killing of Nemo, she fights to protect him and ends up shooting Stan. Stan awakes in the hospital and finds everything is back to normal. He was accepted back into his church when it tuned out that Roger's eggnog "pimp" cup was really the Holy Grail. To protect him from Stan, Hayley had Nemo sent off to Alaska to be raised by Sarah Palin and become the future Anti-Christ.
He is voiced by writer Murray Miller.
- Although his name is Omen backwards, his name is Nemo because Jeff's favorite novel is Finding Nemo.
- Nemo has 662 + 4 = 666 on his scalp.
- Nemo can see through the eyes of animals such as crows and rats, and can also possess other people and turn them into apostates of Satan.
- Nemo displayed telekinesis when he was fighting Stan.