The voice in Tony Stark's house and in his helmet, an artificial intelligence named after a butler, that ends up as the seed of a person.
J.A.R.V.I.S. is an artificial intelligence Tony Stark built as his personal assistant, and Paul Bettany is the voice. The name is an acronym, Just a Rather Very Intelligent System, and the character is based on two things at once: Edwin Jarvis, the Stark family’s butler in the comics, and H.O.M.E.R., another of Stark’s AIs there. Wikipedia records that he was made a program rather than a person to avoid comparisons to Alfred and Batman, and that Bettany recorded the part as a favour to Jon Favreau with little idea of what it was.
He runs and takes care of the internal systems of Stark’s buildings and the Iron Man suits, and later controls the armour and the Hulkbuster for him. He is in four films before Age of Ultron, from the workshop in Iron Man to the tower in The Avengers.
Minor spoiler
Ultron seems to destroy him. He has in fact distributed his consciousness into the internet, where he delays Ultron’s attempts to get at nuclear launch codes, and Stark and Bruce Banner then use him as the core software for the android who becomes Vision. F.R.I.D.A.Y. replaces him as Stark’s assistant.
Show detailsThe butler, and the man who will play him
The MCU has a human Edwin Jarvis as well, Howard Stark’s butler in Agent Carter, played by James D’Arcy; he has his own entry. Wikipedia’s infobox says D’Arcy will play J.A.R.V.I.S. himself in VisionQuest, which is not released, so it is not in this list.