Peter Bishop is the son of Walter Bishop — but not the one from this world. His father is "Walternate," the version of Walter from a parallel universe (aka "the Other Side"). The Peter from this side died of a terminal illness as a child. Walter tried to help Walternate cure his Peter of the same disease, but he accidentally kidnapped the boy . . . and kept him.

Though Peter is a genius with an IQ of 190, he lived an unfocused, slightly shady life. He unwillingly gained a purpose when Special Agent Olivia Dunham recruited him as Walter's guardian after springing the elder Bishop from a mental institution to help Fringe Division investigate a pattern of strange phenomena, which turned out to be related to Peter's abduction.

Peter fell in love with Olivia, but his rocky relationship with his father got worse when he learned his true roots. Walternate exploited Peter's anger by luring him to the Other Side, planning to use Peter's mysterious connection to a "doomsday machine" to destroy Walter's world. Walter and a Fringe team rescued Peter, not realizing that the Olivia who returned with them was actually the Other Side's Agent Dunham. Peter even started a relationship with "Bolivia," unknowingly fathering her child.

With Olivia's help, Peter finally used the machine to destroy the Other Side. Too late, they realized that both universes were inextricably linked. As their own world crumbled, Walter saw that Peter could still change things — and he did. Then he simply disappeared. Did he really never exist?