Myotismon and Henchmen
Digimon Adventure
Myotismon is an enemy with a difference: instead of wanting to take over the Digital World, he wants to take over the real world, so he opens a portal into Tokyo. He also wants to destroy the eighth Digidestined, Kari.
After Gatomon changes sides, she digivolves to Angewomon and defeats Myotismon, though not without the loss of Gatomon's best friend Wizardmon first. When Myotismon rises again in his beast form, VenomMyotismon, the prophecy comes true and WarGreymon and MetalGarurumon defeat him.
More information: The Prophecy
Gatomon
Gatomon was raised by Myotismon, and was his faithful mindslave - I mean henchmon (sorry, I've been watching too much Yu-Gi-Oh! o_O) - until she got to Tokyo. Then she found out that she was Kari's partner - Myotismon wasn't particularly happy about that XD.
Wizardmon
Wizardmon is Gatomon's best friend. He helps out the kids when Myotismon is trying to get rid of them, and sacrifices himself (aww... ;_;) to save Kari and Gatomon.
Demidevimon
Demidevimon is a small bat-like Digimon who hates bats. (Really.) He's Myotismon's right-hand mon, and does his best to stop the kids' crests glowing. Unfortunately for him, he never succeeds and eventually VenomMyotismon eats him.