Accompanying Mio as her personal bodyguard, Yamada is a telepath that once did for a private job management security company what Hotaru does for B.A.B.E.L.—basically detecting and repelling trouble. It is not that out of the question that Yamada was probably prosecuting a security operation in which he was injured and is now Mio's personal bodyguard to repay Mio having nursed him back to health. Yamada's downfall comes about when Sakaki psychometrically detects his back story and thus cleverly takes advantage of Yamada's disorganized thought processes along with his martial arts ability to use adversarial momentum and inertia to his advantage ultimately proving a match for Yamada's reliance upon brute force.

(Source: Wikipedia)