Old Gennai was voiced by Joji Yanami in the Japanese version and Mike Reynolds in the English version.
Young Gennai is voiced by Hiroaki Hirata in the Japanese version and Jeff Nimoy in the English version.
Gennai's first appearance, he is shown to be an old man with a large grey moustache and a nearly bald head, aside from a small ponytail. His eyes are always shut.
In his younger form, Gennai has short, brown hair with a ponytail at the back, and he wears brown and cream-coloured robes.
Gennai is a data-based humanoid being that was part of a group that created the Digimon and Digivices for the Digidestined. Gennai's comrades were slaughtered by Piedmon, who stole the Tags and Crests, but Gennai managed to escape; however he lost the Digivice and Digi-Egg that was supposed to belong to Kari Kamiya.
After the defeat of Devimon, Gennai revealed himself via hologram to the Digidestined and asked them to head to the Server continent to defeat the powerful enemies that resided there.
He would advise the Digidestined during their battles against Etemon and give Izzy's laptop a Digimon Analyzer. Later, he would reveal the existence of the 'Eighth Child', and that Myotismon was trying to gain access to the Real World in order to locate and kill them. When the Digidestined fail to stop Myotismon from entering the Real World and are unable to follow him, Gennai guides them to his home under a lake, meeting the Digidestined in person for the first time. He would go on to tell them how they could return to the Real World to fight Myotismon and find the Eighth Child. He also modifies Izzy's computer further, allowing Izzy to send and receive e-mail from the Digital World.
Gennai's past is revealed when Homeostasis possesses Kari during the Digidestined's fight with the Dark Masters. Gennai would later reveal after the defeat of the Dark Masters that they were not the first Digidestined to come to the Digital World, and that the first group had been summoned to defeat a powerful evil Digimon that came from behind the Wall of Fire, and that the Digidestined were summoned to destroy a new threat, which was revealed to be Apocalymon. After Apocalymon's defeat, Gennai tells the Digidestined to return home, as the portal between the Digital World and the Real World is closing.
Digimon: The Movie
Gennai would later appear to Izzy and Tai to warn them of a threat coming from Diaboromon on the Internet.
Digimon Adventure 02
Gennai called the DigiDestined back into the Digital World to ask them to give up the power of their Crests to add another layer of protection to the Digital World, which they did.
Three years after the defeat of Apocalymon, Gennai would reappear to pass on Azulongmon's DigiCores to the Digidestined. He reveals that he and his kind have been working hard to erase evidence of the existence of the Digital World from the Real World. After the defeat of MaloMyostismon, Gennai reveals that he was in possession of Gatomon's tail ring since she lost it.
Digimon Adventure tri.
Gennai is briefly mentioned by Daigo and Maki as the one who provided the government with the resources to deal with the infected Digimon until they became too strong to handle without the DigiDestined's help.
Digimon Adventure: Last Evolution Kizuna
Gennai appears before Tai and Agumon, only to confirm Menoa's statement to reign true before leaving.