An engineer who went to prison for burgling the company that had been robbing its own customers, and who came out to find the only job available was breaking into somewhere else.
Scott Lang is the only Avenger who joined by accident and the only one who is consistently astonished to be there. He has a degree in engineering from MIT and a burglary conviction for robbing a company that had been swindling its own customers, which tells you most of what the character is: competent, principled in a way that is inconvenient, and bad at the paperwork of being a person.
What the films use him for is scale, in both senses. He is the small one in a story about gods, and he is also the only main character whose central problem is custody of his daughter. Those two things are the same joke and the same argument.
He is also, structurally, the reason the last film of the Infinity Saga happens. Everyone else has run out of ideas. He walks up to the gate with one.
Show detailsA rat
The mechanism by which he leaves the Quantum Realm is a rat walking across a control panel in a storage unit. Five years of grief for the rest of the cast, five hours for him, undone by a rodent.
The films are entirely serious about this and it is the closest the Infinity Saga comes to admitting how much of it turns on luck.