Hachiman is the Shinto god of war, and divine protector of Japan and the Japanese people. The name means God of Eight Banners, referring to the eight heavenly banners that signaled the birth of the divine Emperor Ōjin. His symbolic animal and messenger is the dove.
"The guardian god of the samurai. Of Japan's many gods, he is the most revered. According to the history of the Usa Shrine, Emperor Ojin is Hachiman. But it is also said that Hachiman is a legacy of Taoism, Buddhism, and other mainland cultures, which shrouds his origin in mystery. Since he was the Genji clan's guardian god, he has been worshiped by samurai since the Kamakura era."
—Soul Hackers 3DS Compendium
A Hachiman is necessary in order to fuse Zeed into the generals Yoshitsune (if Zoma is level 60 - 79) or Masakado (if Zoma is 80+).