Vicious

Vicious (Bishasu in Japanese) is a Team Rocket member, known as the "Masked Marauder", supposedly a high-ranking Rocket. Making his debut in the 4th movie, Vicious threatens a poacher (who encountered Celebi years ago) and actually steals his Pokemon until the poacher reveals Celebi's location.

The rumor floating around for awhile was that Vicious, despite his obvious ugliness, was really the son of Jessie and James, but that rumor has been proved false. In fact, what I've heard is that Vicious has "broken the Rocket bishonen code" (bishonen being an attractive guy). There is no mention of this in the English version of the movie, however.

Vicious uses a TR-inspired technology to steal Pokemon - Dark Balls! These devices not only steal Pokemon (whether they are trained or wild), but also turn them *evil*. He uses one of these Dark Balls to capture Celebi and make it destroy the forest using a huge mech made of trees and leaves.

Celebi summons a giant windstorm to gather materials for the mech, and accidentally kidnaps Jessie. Vicious won't release her, saying he needs someone to witness his power. Jessie tries reasoning with him, but to no avail (Vicious claims that Giovanni will soon be answering to HIM.)

Of course, Celebi comes to its senses after remembering the 'fun' it had with Ash and Sammy, and stops being evil, yada yada. Vicious is ultimately thrown from the mech, his mask comes off, and the forest Pokemon deal with him...

Vicious has the following Pokemon: Tyranitar, Houndoom, Sneasel, Scizor, Scyther, and Celebi. Most, if not all, of these Pokemon were stolen using Dark Balls.