He is Ben's partner and most trusted friend.
He is a common species of Gabumon, the type that are only called "Gabumon." Ben found him as he was searching his forest home in the digital world (for something he can't remember) two years ago, recently hatched. He lived with Ben for almost two years before the two were sent back into the digital world.
He earned his name, Fang, from Ben when Edward tried to kill Danielle. Ben, at that time, was so angry he he spouted randomness...And the name stuck.