Sepek is one of the Vulcan children in Yesteryear who mock the seven-year-old Spock for his human blood and jeer that he will never be a true Vulcan.
Sepek appears in the Animated Series episode Yesteryear, in the scenes set in the city of ShiKahr some thirty years before the main story. He is a boy of about Spock's age and the leader of a small group who wait for him outside his family's home.
The boys taunt Spock as an Earther, insult his father for marrying a human and say he cannot even manage a proper nerve pinch. Spock, who has not yet mastered his feelings, loses his temper and goes for Sepek, which only proves their point.
The adult Spock, who has returned through the Guardian of Forever posing as a cousin named Selek, watches the scene. The bullying helps to push the child into his early attempt at the kahs-wan ordeal in the desert, where his pet sehlat I-Chaya is fatally hurt.