Reformatted from an Apocalymon, Mephistomon has a deep hatred for all life, and so plots to destroy the world and everything in it. In order to do so, he disguises himself as Ryuuji Tamashiro, the president of the Okinawan VP Laboratories. He soon becomes famous for his part in marketing the V-Pet, a digital pet that he modified to cause other Digimon to Bio-Emerge and become his servants.
Takahito Uehara, the original creator of the V-Pet, detects the virus that Mephistomon had implanted in the V-Pets, and creates a vaccine program which he implants into the original V-Pet, which manifests as Seasarmon. When Mephistomon finds out about the vaccine, he uses his servants to kidnap Takehito and find out the vaccine's location. Once Uehara reveals that the vaccine is in Seasarmon, Mephistomon doffs his disguise and attacks Seasarmon and the Tamers. As part of the battle, he drags the group into an apocalyptic dimension modeled after his intentions for the Real World. Even Mephistomon was stronger the Tamers' digimon (who fought in the their Champion forms), he was defeated by Growlmon blasting a ship floating in the sky, causing it to fall on him, allowing the vaccine program to activate and purge the virus. It is revealed that Mephistomon is not actually dead, and he digivolves to Gulfmon.
The name "Mephistomon" refers only to the first Ultimate form of this Digimon. Throughout the series, Mephistomon gains the ability to Digivolve into a number of other forms, each with a different name and special attack. Mephistomon, however, is the form he spends most of his appearance in.
Apocalymon attempts to Bio-Emerge in the Real World, but is confronted by Omnimon. During the battle, Apocalymon reformats to Mephistomon. Omnimon attacks him, and even though Mephistomon was no-match for him, Omnimon is unable to stop Mephistomon from Bio-Emerging.
Gulfmon is Mephistomon's larger and more powerful but less intelligent form. The Tamers' Champion Digimon are unable to scratch him, but Calumon's powers activate, digivolving them to Ultimate. As Gulfmon charges at them, they combine their powers into their "Trinity Burst" attack, impaling and destroying Gulfmon.
Role in the series
Mephistomon as KOMPLEX, is revealed that he is really the computer program form of the Virus user A'Bu-se, a corrupted Utrom who created Komplex to decieve the toads of their homeworld to do the work of the toads and learns the dark arts of fusing his soul to the deceased Dark Spawn Lord, Mephistomon. Once the Komplex program had done the duties of controlling interest rate and all the boring things the toads never wanted to do, A'Bu-se used his invention "mind control transferator" and fused his mind to the Komplex program and became the Komplex program itself for A'Bu-se's commands from Kin Einmortal to conquer all systems in the fan universe, starting with Earth. But after his defeat in the first 3 episodes of the Dave Felis series, it was revealed that he was merely a Cyran servant to an even greater Dark Spawn Lord called Kin Einmortal, leader of a separate group of villains known as the World Corrupters. Upon this revelation, he destroyed the entire Toad Empire, eliminating the toadians forever, and leaving the Toad Generals stuck on Neverland while he joined up with his true master. After A'Bu-se revealed his true self to the other servants of Kin Einmortal, he ordered the Komplex program's old Transmentalfusion Device to be tossed into the ocean, presumed this Komplex form might never be seen again, and it ain't known it will resurface because of Kin Einmortal's orders to abandon it and the former demon beast army to Kin Einmortal. In other words, Komplex is not a true darkspawn, turning out to be a suit for an eviler alien who had to use Mephistomon's soul to reclaim his master's work.