Gaster

Gaster was the Royal Scientist of the underground, before he fell into his own creation and was shattered across time and space. He tried to escape with the help of the Fates, but was betrayed. He tried to get revenge later on, but was ultimately killed by Yubam Endall.

As the Royal Scientist
Gaster experimented with Determination in the Undertale universe. He built a Determination Extraction Machine, and also the Core. Some time later, he fell into his own creation and was shattered across time and space.

Alternate Timeline
While Gaster was shattered, Daniel found him and granted him the power to return to the Undertale universe. Gaster returned, and killed everyone to absorb their power. With this power he traveled to an alternate Pokemon universe using the Portal of Worlds, but was defeated by the player and sent back into nonexistence.

Gaster's Revenge
While Gaster was shattered, he found Daffy and started whispering to him about the Fates, and how they were the source of all evil in the world. He tried to use Daffy to destroy the Fates with the Soul Infinity Stone, but his plan failed when Daffy was defeated by the player.

After Chara took Daffy, Bugs, and the player into a pocket universe, Gaster decided to show his face and take on the player himself. After he lost, Daffy teleported Chara, Bugs, and the player out of the universe and Gaster was destroyed by Yubam Endall while swearing revenge.

Powers
Gaster doesn't have much power of his own, but he has created Gaster Blasters as a powerful form of attack.

Gaster Blasters
Gaster can summon Gaster Blasters to blast a beam of energy at whatever he wishes to target.

Karmic Retribution
Gaster can use the power of karma to "poison" his enemies.

Trivia

 * The reason Gaster was able to find his way into the alternate Pokemon universe was because Daniel let him in withe the Rule of Cool.
 * When Gaster swore revenge, he actually swore revenge. As he was dying, he said "UNTIL THEN, YOU CAN GO TO HE-".