Mecha-Ude: Mechanical Arms (Volume 1)

Kita-Kagami City is a pretty normal town... except that some people have gotten their hands on Mecha-Ude: powerful, sentient, mechanical beings that attach themselves to their human hosts and have awesome capabilities. When middle-school kid Hikaru accidentally activates Alma, a Mecha-Ude with no memory of its past, they must team up to uncover the mystery of Alma's identity and prevent the powerful Mecha-Ude from ending up in the wrong hands.