Gennai is the mysterious guide of the digidestined. He first appears in a light hologram after the defeat of Devimon, to inform the children of their tags and crests, and evil enemies waiting to be defeated on other islands.

Who or what Gennai is was never really explained. He is not a Digimon, but he lives in the digital world and can't/won't go through the gate to the human world. He has been living in the digital world for a long time, and knows a lot about it and other worlds. He occasionally shows up to give clues to or otherwise aid the digidestined.

He appears as a hologram screen image a few times before the children finally meet him in person. He gives them Digimon keycards and tells them they can use them to open the gate to the human world.

Gennai lives in a house inside/underneath a lake. It's a pretty nice house. He's quite smart with computers and electronics. Izzy looks up to him, but the others are a little suspicious of him, or annoyed by him. Then, it is Izzy he helps the most.

Gennai's personality... he's a helpful guy but has a tendency toward absolutely terrible jokes. He put weird things on Izzy's computer. ^^;; He also has a tendency to cut out at important times in his transmissions. He's very knowledgeable and kind of helps to look after the digital world.

He has been in the digital world for many years, it seems. Back when he was young (and, oddly, a lot taller), he was part of a group that had developed and looked after the children's digivices, crests and digieggs. When Piedmon attacked, Gennai alone survived and escaped, taking the digidestined treasures with him.

It is Gennai who tells the children about their crests, about the existance of an eighth child and produces the prophecy which enabled the children to defeat VenomMyotismon. Among other things. Basically, when there are things and plot devices beyond what Izzy could find out on his computer, Gennai steps in to provide the info.

In 02, he appears as a younger, less ugly man, and does some stuff. I can't be any more concise than that (which is rather sad, really ^^;;) because I haven't seen any of the eppies in which 02 Gennai appears, 'cept the last one which was in Japanese but this is an 01 profile page so mreeh.