B't X

Overview

Kotaro is a robotics and technology genius who has become the object of an evil group's attention. They want Kotaro to help them build their Machine Empire and beef up their artificially intelligent creations, the B'ts (pronounced "beats"). Once the Empire captures Kotaro, he may have no choice but to work for them, unless his kid brother Teppei can somehow rescue him. But after a powerful B't and its master attack Teppei, it appears Kotaro's last hope is lost. However, Teppei's spilled blood awakens a legendary B't (these evil creatures bond with blood), so Teppei now has one of the most deadly, sentient creatures-the B't X-at his command. Still, the brothers are up against an army from the Machine Empire, and the odds are against them. Even if they survive, B't X isn't keen on going up against another creature from the Machine Empire. (Source: TOKYOPOP)

Source: AniList