Arima

Arima is a major character in the Pokemon Saga. She lives in the Pokemon Universe, and has extremely powerful psychic/magic abilities.

Origin
Not much is known about Arima's "origin story", other than she trained Twilight Sparkle as an apprentice and helped Cyrus escape the Distortion World, where he would have been residing for the rest of his life.

And, at some point, she gained the power to take on a different form that makes her Pokemon's powers much stronger, even to the point of Mega Evolving without the stones.

First Apperance
Arima first made an apperance in the Sinhoh region, where she told the player about the True Nature of the Universe, and how weird the game is. She guided the player through her tower until eventually the player found Albert.

Gym Leader
Later on, she became a Gym Leader in the Bisque region in case anyone came across it. She also let King Dedede and Trebla live in her tower there, as Gym Leaders.

Albert's Plan
When Albert tried to get Discord to convince the residents of Johto that it is just a game, she helped the player find Discord and put a stop to his chaos, by following them around and healing their Pokemon.

On Soul Island, she is there on a mission for the Phoenix Foundation to stop Rainbow. She transformed into a boulder, but alas, she was literally pushed aside by the players' Pokemon.

On the top of Soul Tower, after Rainbow opened up a portal to the Void, she and Phoenix came to the player's aid, only for Phoenix to get shot. She healed the player's Pokemon before the battle with Li'l Gid Gid.

The Dedede Case
After Bill Cipher was defeated by the player, she looked for King Dedede in the Psychic Tower, but he wasn't there. She then realized that Li'l Gid Gid had stolen that Dedede, and now she'll never seeis.

Powers
Arima has a lot of special powers, some of which she was born with, and some of which she acquired later. She seems to be constantly trying to improve these powers, meditating very often.

Shape-shifting
This is the power she uses the most; she can shape-shift into anything else, and she can even shape-shift others, to an extent.

Telepathy
This power allows her to speak to others in their minds, and to an extent, control other people.

Reality Warping
She has slight reality warping abilities, as shown in the Sinhoh Region where a floor of her tower is really messed up.

Teleportation
Arima can teleport to other locations quite easily, but not if they are too far away.

Mimikyu
Probably because of Arima's ability to shape-shift, Mimikyu is her favorite Pokemon. She doesn't usually use her Mimikyu in battle, and she cares about it very much.

Mimikyu's moves are: Shadow Claw, Play Rough, Double Team, and Mimic.

Her other Pokemon are Drifblim, Alakazam, Gengar, Gardevoir, Espeon, and MISSINGNO.

Trivia

 * The name Arima comes from the Basque origin. In Basque the meaning of the name Arima is soul.
 * This name was chosen because the first full game she appears in is called Pokemon Bisque, which is only one letter off from Basque.
 * Ironically, Arima never had anything to do with the Soul Gem in any of the games.