Ten
Information
Jariten, known to everyone as "Ten" is Lum's little cousin. Ten is a bratty and smart-mouthed alien toddler. Like all oni, Ten has powers of his own. He can fly and breathe fire. But he is an inexperienced child so his flying is awkward and slow, and his fire breathing isn't always effective.
Ten is a child, but sure doesn't act his age. He has a certain level of adult intelligence which is quite shocking for someone still in diapers. One result of being precocious is that Ten only has eyes for older women. Yet Ten is still essentially a child and quite naive. Around women he is usually cute and diminutive. When he's near men he's his usual self. Loud, pesky and often aggressive.
Ten came to earth to meet Lum's new husband but was disappointed at what he found. From the moment they've met, Ataru and Ten have been at each other's throats. Because Ataru is such an idiot, Ten doesn't think he deserves Lum. Therefore he decides to live with them to keep an eye on Ataru.
(info from Furinkan.com)
