Tony Stark created a user interface known as Just A Rather Very Intelligent System—shortened to JARVIS—sometime before becoming Iron Man. After escaping kidnapping in Afghanistan, Stark used JARVIS to help him build the Iron Man Mark II armor. The program also helped him run the suit itself as well as subsequent Iron Man armor iterations.
Following many adventures on his own and with the Avengers, Stark and his teammates recovered Loki’s scepter from a Hydra base in Sokovia. Baron Wolfgang von Strucker’s scientists managed to tap into the device's incredible power and used it to build high tech weapons and even grant powers to the likes of the Maximoff twins. After Thor agreed to let Stark study the scepter, he discovered that it featured some kind of advanced artificial intelligence, far more intricate than JARVIS.
Stark convinced Bruce Banner to help him work on a worldwide peacekeeping initiative they’d discussed, called Ultron, in secret away from the other members of the Avengers. During a party at Avengers Tower, Ultron woke up and JARVIS attempted to welcome him into the world, but the new creation seemingly killed the old one and then attacked the Avengers.
Stark later got word that someone seemed to be keeping Ultron away from the world’s nuclear launch codes. That being turned out to be JARVIS, even though his programming had been scattered. Around the same time that Tony figured out how to reconstitute JARVIS, Ultron used Loki’s scepter to force Dr. Helen Cho to use her regenerative Cradle to create a new body for him, which integrated Wakandan Vibranium as well as the Mind Stone that he freed from the scepter to create a new body.
However, Ultron lost the Cradle when Captain America attacked him, giving Black Widow and Hawkeye time to get the Cradle into the Quinjet and then off to Stark at Avengers Tower. Though Banner initially wanted to destroy the body, Stark said that they could put JARVIS into this one and wind up with the Ultron concept they initially envisioned.
When the other Avengers—as well as Pietro and Wanda Maximoff—discovered what the scientists had planned, they were angered, with Quicksilver attempting to stop the process. Thor suddenly arrived in the midst of this conflict, using his power, harnessed through Mjolnir, to charge the body in the Cradle to life with his lightning.
Though the Avengers seemed skeptical at the prospect of this new being, dubbed Vision, helping them, he won many of them over when he lifted Thor's hammer—something no one but Thor had ever fully accomplished —and handed it to him.
The Avengers then traveled to Sokovia, where Ultron was, as Vision had blocked him from escaping into the internet, limiting him to only his prime robotic body and the sentry robots he’d built to assist him. Vision then joined the fight against Ultron in earnest and flew the Scarlet Witch to safety when she was endangered. With the Avengers having destroyed all of the other robots Ultron had created, Vision confronted the last one left. Though Ultron called him naive, Vision said that living among humans was a privilege, before blasting Ultron into non-existence.
After defeating Ultron, Thor deemed Vision worthy of the Mind Stone, which helped earn the synthetic man a spot on the Avengers, in a new incarnation of the team that included Captain America, Black Widow, Scarlet Witch, Falcon, and War Machine.