Domu (ドム, Domu) and Yu (禹, Yǔ) are a minor ancient mamodo team who only appears once. As their spells are
unnamed, their spells are based on Sand. By the anime you can tell he is quite reckless as he desperately charges towards Brago without even using a spell (episode 77).
Yu seems to be a Boy whose mind is brainwashed by Zofis, but got free once Brago burned all the ancient mamodo books, including his book.