Ghetsis

Ghetsis Harmonia, or simply Ghetsis, is the main antagonist in Pokemon Black and White, as well as the adoptive father of N. He is one of the Seven Sages of Team Plasma. All along the story, he pretended to serve N and to fully support Team Plasma's utopy, though he often lets slip that he serves his own interests. In fact, his true goal is to be the only person in Unova to have Pokemon, so that he would be able to take over the entire land. To do so, he manipulated his own son as a puppet-king of the Team to make him capture Reshiram or Zekrom, so that he would be able to use them.

Pokemon Black & White
He was first seen in Accumula Town talking to the locals that they shouldn't enslave Pokemon. You encounter him several times telling everyone to send their Pokemon to Team Plasma to be released back into the wild.

After the player beats N in his castle, Ghetsis will come up and say that N was a disappointment losing to the player, and it was revealed that Ghetsis was using N for his own evil deeds, calling him a good-for-nothing boy, warped, and defective. After that, he will battle the player in a rage. When defeated, he will be taken away by Cheren and Alder. After the game is completed, it is revealed that he escaped thanks to the help of The Shadow Triad, his most faithful and loyal servants.

Black 2 & White 2
In Pokémon Black 2 and White 2, Ghetsis is the leader of a new Team Plasma. This version of Unova's villainous team is more straight forward with their plans to rule Unova  by freezing the region rather than freeing Pokemon. He has recruited Colress and Zinzolin to help him.

He first appears at the Giant Chasm where he announces that he will awaken and use Kyurem to freeze the land. Upon being encountered again, heorders Kyurem to kill the player, but N destoys the attack. Ghetsis fuses N's dragon with Kyurem, making it even more powerful and forces the player to battle it. Kyurem escapes, and the player must battle Ghetsis. Upon losing, he suffers a mental breakdown and is escorted away by a member of the Shadow Triad saying he has "lost control".

After the game is completed the Shadow Triad are found Icirrus City. They state that Ghetsis is no longer capable of doing anything let alone commit further criminal acts, implying that this loss of control may be permanent.

In the anime
Ghetsis made his debut in BW112, where he was seen briefly contacting Colress via a hologram communicator. He is the leader of Team Plasma. He is set to make another appearance during the climax of Episode N.

He appeared again, this time in person, in BW117. He was seen contacting Colress for an update on his Pokémon control experiment. Later, he spoke to several Team Plasma Grunts, who were spying on the excavations made at the White Ruins as well as looking for N and conducting the mysterious "Project G" and "Project F."

Ghetsis appeared in BW118 discussing his plans with Colress and Angie. He also was seen in flashbacks to N's past, in which he raised N alongside Anthea and Concordia and wild Pokémon. In a later flashback, a ceremony he led in N's Castle involving N and the Seven Sages was interrupted by Reshiram, infuriating him.

He also had a brief appearance in BW119, meeting with Colress, Angie and Brad via hologram and revealing the existence of the Light Stone to them.

He appeared briefly via a communicator BW120 before arriving at the White Ruins in BW121. With his followers having subdued Team Plasma's enemies, Ghetsis took the Light Stone and placed it on a pedestal in the ruins, then used his staff to initiate the ritual leading to Reshiram's awakening.

Ghetsis appeared in BW122, where, with the help of Colress's machine, he took control of Reshiram. Despite turning it on his enemies, however, he lost control of Reshiram when Colress's machine was destroyed. Ultimately, he was arrested by Looker along with the rest of his followers.

Pokémon Adventures manga
Ghetsis makes his first appearance in Accumula Town, where he introduces Team Plasma and their campaign to the townspeople. He gives a speech asking the townspeople to free their Pokémon so that they can be truly equal to them. Although some were not swayed by his words, Ghetsis did manage to convince some of the townspeople to release their Pokémon into the wild.

During the attack on Nacrene City for the Dark Stone, Ghetsis had a Plasma Grunt trick Black into revealing where it was kept hidden. After reaching the room, Ghetsis attacked Black and paralyzed him with his Eelektross. With the Dark Stone now in his possession, Ghetsis delivers the stone to N so that he can awaken the legendary Pokémon Zekrom.

When Team Plasma attacks the Pokémon League, Ghetsis uses Hawes as hostage to prevent Brycen from freeing the kidnapped Gym Leaders. He puts Brycen and the other Gym Leaders on crucifixes to taunt Black and others. Black attempts to retaliate, but his entire team is easily defeated by Ghetsis's Hydreigon. When no one is able to stop Team Plasma, several Trainers that Black met on his journey offer to help combat Ghetsis and the other Sages so that Black can move on.

His Volcarona was found in the Relic Castle, where it was later captured and eventually is the mother of the Shadow Triad's Larvesta.

Trivia

 * His strongest pokemon is a Hydreigon at level 54. However Hydreigon's previous evolved form; Zweilous evolves into Hydreion at level 64, so his Hydreigon is underleveled.
 * Ghetsis is the only antagonist that uses a Pseudo-Legendary Pokemon during the final battle with the main hero.
 * In Pokémon Black 2 and White 2, Ghetsis becomes the first Trainer in the main series games to use seven Pokémon to battle, due to controlling Kyurem and having a party of six Pokémon. Overall, he is the second Trainer to do so, the first being Greevil in Pokémon XD: Gale of Darkness.
 * The Ghetsis Stick disables Poké Balls, which he uses to stop the player from catching Kyurem while it is under his control.
 * In Black & White, all his pokemon are at level 52 and his strongest (Hydreigon) is at level 54. In Black 2 & White 2, his pokemon are at level 50.
 * His attempting to freeze the player alive was often mistaken on the internet as trying to impale them with icicles, before the translated versions were released.
 * On the Internet, he is known as "Dennis" due to people mishearing the chanting happening during the battle with him. It can be heard at about 1:32.
 * His last name is Latin for "harmony".
 * Ghetsis is the only villainous team leader in the anime to be arrested successfully at the conclusion of his plot.