According to Nico, Yamada is a son of a automobile businessman. He is an environmentalist who wants to dismantle their country's automobile industry, in which George refers it as a loony policy.
Nico learned about Yamada when she started to investigate about the costume killer. Yamada is killed by the hands (or rather flippers) by a giant emperor penguin.
It is revealed in Imelda's secret message that Arno and Yamada visited Paris in the second week of July at the same time and implied that they were the members of Neo-Templars. Imelda warns Plantard that they're becoming the assassin's next target.
Yamada is a common Japanese name meaning "mountain ricefield".