Ten is a young Oni from space and Lum's cousin who arrives on Earth to evaluate her chosen partner, Ataru Moroboshi. He quickly judges Ataru unsuitable and resolves to end their relationship, moving into the Moroboshi household under Lum's guardianship.

He has green hair, a small horn on the back of his head, and typically wears a yellow tiger-striped diaper. Ten can fly, though not at high speeds, and breathes massive amounts of fire—a tendency that verges on pyromania.

His personality is bratty and mischievous, especially toward male characters like Ataru, whom he often torments with fire attacks, earning the derogatory nickname "Jariten" or "Bratty Sky." Around pretty girls, however, he adopts a false innocence to win their affection, displaying behavior surprisingly similar to Ataru's lecherous tendencies. He shares a close familial bond with Lum, trusting her more than his own mother and relying on her for protection, while also acting as her informant by spying on Ataru.

Ten's mother is a firefighter with a deep passion for her job and a strict stance against pyromania. He fears her disapproval and hides his fire-breathing when she visits Earth. She has given Ataru a fire bell to summon her if Ten causes trouble, which Ataru uses to bully him.

He maintains a strong friendship with Kotatsu-neko and developed a romantic crush on the school nurse, Sakura, once asking her on an unreciprocated date. He becomes envious when Ataru flirts with Sakura or his own mother.

Ten speaks with an Osaka dialect and is often called "Ten-chan" by female characters and "Jariten" by the males. He is unable to swim.