The Jak and Daxter universe is populated primarily by a humanoid elfish species. Jak is the main protagonist of the series and is playable in every game with the exception of Daxter. In The Precursor Legacy, he accidentally turned Daxter from a human into an ottsel (a fictional hybrid of an otter and weasel) after accidentally bumping him into a reservoir of Dark Eco and sets out on a quest in attempts to undo the transformation. In Jak II, he is infused with Dark Eco due to experimentations performed by Baron Praxis, and in Jak 3 he is blessed with Light Eco by the Precursors. Both Light and Dark Eco abilities are expanded upon during the course of the story in “Jak 3.” In Jak X, he is forced to race in the Kras City Grand Championship in order to save his life. He later travels to The Brink to investigate the Eco shortage that is ravaging his world in The Lost Frontier.

Daxter is Jak's sidekick and is often found getting into trouble. Daxter was once a human but fell into a pit of Dark Eco in the opening cutscene of the first game in the series, which mutated him into an ottsel. He rescues Jak at the beginning of Jak II and becomes playable intermittently for the first time in the series. In Jak 3 his role is expanded, and he gets a pair of pants as a reward for his efforts as an ongoing gag in the game. In Daxter, he details his adventures in the two-year time span before he managed to break Jak out of prison.[21] In Jak X, he accompanies Jak and his friends to Kras City, and assists them as they compete in the Kras City Grand Championship. He later travels with Jak and Keira to The Brink in The Lost Frontier.