Vision

Vision is a synthetic being created by Ultron. He turned on his creator and joined the Avengers.

Biography
Vision was created by Ultron in order to build an indestructible body for it. This was to be Ultron's "vision for the future." He was tasked with stealing adamantium from Weapon X and Captain America's shield as well as vibranium from Wakanda.

Vision successfully infiltrated Weapon X but failed to acquire the shield and vibranium. While in Wakanda, he was damaged by Black Panther, Captain America, Thor, and Hawkeye. Vision retreated after being defeated.

Ultron was upset at his creation's failure but was fine with just having adamantium.

Vision built synthetic versions of each of the Avengers so they would replace the real ones. However, Vision's artificial intelligence began growing outside Ultron's original programming. He began question his creator's actions.

Captain America infiltrated their base and Vision was ordered to kill him. While battling, Vision asked why he kept fighting. Cap replied that it was important to be human. Soon after, Vision betrayed his creator just before Ultron could kill Cap and freed the captured Avengers. He exploited Ultron's one weakness by disconnecting the head from its body allowing the Avengers to defeat him. While fleeing the base, Cap saved Vision.

Upon their return to the Avengers Mansion, Vision explained that he came to the conclusion that Ultron's actions were incorrect. Ultron had programmed him to be perfect, but he believed he should be more like humans.

Vision attended the funeral of Hank Pym, his "grandfather." Afterwards, a new hero named Yellowjacket appeared. The team then discovered that this was a more aggressive Hank. He joined the team, ending his resignation.

Vision went down for repairs and woke up a month later. However, he found that the team had become dictators, ruling the world. He discovered that Purple Man used his powers on Iron Man who used his satellites to amplify his powers. Vision was able to snap the team out of their spell and free everyone.

He joined the team along with the New Avengers in defeating Galactus and his heralds.