Mecha-Ude: Mechanical Arms (Volume 2) by Yoshino Koyoka (author and illustrator)

The action continues in volume 2! An ordinary middle-school student’s life takes a thrilling turn when he encounters a powerful mechanical being in this exciting new manga series for fans of My Hero Academia and Spy x Family.
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 robotic arm Mecha-Ude that has no memory of its past, they form an unlikely bond. But where did Alma come from? And why are shadowy secret organizations suddenly chasing Hikaru, stopping at nothing to steal Alma?
Together, Hikaru and Alma must uncover the truth behind the Mecha-Ude’s identity and prevent it from ending up in the wrong hands.