As a form of Apocalymon, Mephistomon has a deep hatred for all life informing his plot to destroy the world and everything in it.
Having reformatted itself from Apocalymon, Mephistomon is too weak to defend itself from Omnimon's onslaught, but is small enough in data size to escape to the Real World by bio-emerging. In order to feed his thirst for destruction, Mephistomon disguises himself as "Ryuuji Tamashiro", the president of an Okinawan company named "VP Laboratories". When Ryuuji, masquerading as a prisoner of the Digimon who are in fact his own servants, learns that Minami's Digimon Seasarmon is actually the vaccine designed to thwart his plans, he triumphantly doffs his human disguise and attacks Seasarmon and its friends in his true form.
He drags the group into an apocalyptic dimension modeled after his intentions for the Real World. Even though Mephistomon was stronger than 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 has instead digivolved to Gulfmon.
Gulfmon is Mephistomon's Mega and true 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 Force" attack, multiplying their combined strength and generating enough power to impale and truly destroy Gulfmon.
Ryuuji Tamashiro (玉城 隆二 Tamashiro Ryuuji?) is Mephistomon's human form, a disguise he took to implement his plan for destruction. Ryuuji is the president of an Okinawan company named "VP Laboratories", which produces a digital pet toy named the "V-Pet". As the president, Ryuuji implanted a virus within the V-Pet which secretly allows other Digimon to Bio-Emerge in order to wreak destruction and act as his servants, and Ryuuji quickly gains fame for his part in marketing the toy. However, the original creator of the technology, Takahito Uehara, detects the virus and creates a vaccine for it which he implants in the original V-Pet program, which he had given to his daughter, Minami, as a replacement for her deceased pet dog Mei.
When Ryuuji learns of the vaccine's existence, he signals the corrupted V-Pets to begin attacking the Network and sends his servant Digimon to recover the vaccine by kidnapping Takehito, his daughter, and the original program, while simultaneously masquerading as just another prisoner of the Digimon, held captive in the same building as Takehito and Minami. The captives are nearly rescued by Takato, his cousin Kai Urazoe, and Growlmon, who had befriended Minami before she was kidnapped, as well as Seasarmon, who had bio-emerged from Minami's V-Pet, but when Takehito explains that Seasarmon is the vaccine necessary to calm the rampaging Digimon, Ryuuji triumphantly doffs his disguise and attacks the group as Mephistomon.