Ultron (2010 Marvel Animated Universe)

"I've hoped humans and Inhumans would annihilate each other for me, but if I want the job done right I'll have to do it my self. Now a new world order, THE ULTRON REVOLUTION!"

- Ultron revealing his true colors to the Avengers. "Your father's creation has no limits to how much power it can absorb, so now, neither do I. Prepare for humanity's end."

- Ultron's first quote.

Ultron is a major antagonist in Avengers Assemble, serving as a major antagonist in the second half of Season 2 and the main antagonist in Season 3 (entitled Avengers: Ultron Revolution).

History
Ultron first appeared in the show by possessing the body of the robot Arsenal after the Avengers defeated Thanos, which was due to the backdoor left open in his programming. Using Arsenal's energy absorbing powers, he completely drains the five Infinity Gems, while quickly defeating the Avengers and leaves. Then Hulk jumps in, trying to attack Ultron, but Ultron quickly uses the Power Gem to attack Hulk, then Iron Man and Captain America. It is possible that the Avengers have fought Ultron prior to the beginning of the series as all of them (except for Sam) weren't scared to see him.

After possessing Arsenal, Iron Man, guilty over Ultron's possessing of his robot best friend, tries to locate Ultron using his "Ultron Detector App". Unfortunately, with the obsession, makes his leadership become risky, and as a result, cause the team to be split right down the middle. Unbeknownst to any of them, Ultron engineered this rapid distrust as part of his plan to destroy the Avengers from the inside out. He then sent a Hammer Industries robot to attack the Avengers as they were celebrating Thanos's defeat. After they destroyed it, Ultron then took control over Justin Hammer's technology and entrapped him with an experimental suit of armor that he recently made. After the Avengers were able to destroy the robots, they located him in an abandoned A.I.M. facility, however Ultron was already ahead of them and ambushed them with the facility's hidden weapons. This also furthered the distrust and suspicion between the team, not to mention towards Iron Man, as they thought he knew this was coming. The weapons were destroyed by them, and Iron Man opted to go ahead and take down Ultron. Iron Man found Ultron upgrading himself with the stolen technology from Hammer and A.I.M., as well as fix the "flaws" that Arsenal seemingly possessed. He then battled Iron Man, and Ultron soon proved to have the upper hand, almost killing him, and ripping his armor apart, piece by piece. Luckily, the Avengers stepped in and battled him while Iron Man retrieved his weapon to destroy Ultron's connection to Arsenal. Just as he was gonna finally kill Ultron for good, Iron Man took a risk by shutting off Ultron's connection to Arsenal, but that risk failed, and Ultron escaped. Angered by these risks, Captain America quit the team because he felt he couldn't trust Iron Man anymore; just as Ultron predicted.

Ultron made his next move by possessing the Super Adaptoid, and then attacking the Roxxon corporation, just to draw the Avengers out. When the Avengers came in there, along with Spider-Man, Iron Man revealed that he knew that Ultron was possessing the Adaptoid all along. While fighting, Cap, who had just joined S.H.I.E.L.D., entered the fray to deal with Ultron himself. Of course, the team distrust continued to widen while Ultron hijacked Cap's communication uplinks to bring on his LMD army. He then proceeds to infect his LMDs and turn them into his Ultron sentinels. While the distrust continues and they were forced to battle the LMDs, Ultron then used the uplink to trick S.H.I.E.L.D. into attacking the Avengers, under the pretense that they were the rogue LMDs. After they shut down the Adaptoid, Ultron escaped, and then went back to his prime Arsenal body. He then took control over J.A.R.V.I.S., and the entire Stark armory, and had them fight the Avengers. Iron Man had to shut down his entire tower to force Ultron out. Ultron once again escaped, this time into space. There he witnessed the Avengers split apart, just as he expected.

He the turned himself into a nanovirus, and attacked the Avengers yet again, infecting Falcon. The Avengers were forced to fight Ultron/Falcon, but their distrust and uncoordination as a team proved to be a weaknesses. The Avengers decided to work together for old times' sake to deal with Ultron. Unfortunately, by then, Ultron had already infected the city with his virus. When Falcon managed to fight Ultron's control, he tells the Avengers that they need to destroy Ultron once and for all, and Arsenal with him. Iron Man reluctantly decided to do it, and confronted Ultron, and then flew him out to space, all the way to the Sun. Meanwhile, Falcon weakened Ultron by transferring all of the Ultron copies into his primary body. When Ultron tried to escape, Iron Man personally rocketed him out to space. While doing so, Arsenal managed to come back, and after apologizing to Iron Man, sacrificed himself by rocketing into the Sun and take Ultron with him. Ultron screamed in agony as he gets burned down by the Sun.

As it turns out, Ultron was somehow recovered by A.I.M., and as a result, Ultron came back to life, and killed and absorbed the A.I.M. Scientist Supreme into himself, creating his new body, much to the Avengers' abject shock and horror.

Trivia

 * His secondary form is a tie into the film Avengers: Age of Ultron.