A mercenary with terminal cancer takes an experimental cure that works, leaves him unable to die and unable to look like himself, and turns him into the only person in these films who knows he is in one.
Wade Wilson is dishonourably discharged from the Canadian special forces, works as a freelance mercenary, meets Vanessa, proposes, and is diagnosed with terminal cancer, roughly in that order. The man who offers him a cure is running a programme that tortures people until their latent mutant genes activate, and it works on Wade in the worst possible way: it takes the cancer and leaves the scarring, permanently.
What separates him from everyone else in this catalogue is that the healing factor is not presented as a gift. It is the reason he cannot die when he wants to, the reason grief has nowhere to go, and the joke and the wound in the same place. The films are extremely funny and they are, structurally, about a man who has been denied an ending.
He also talks to you. He knows the budget, he knows which actors were available, he knows what studio owns him. This is the one character here whose continuity errors are jokes he makes on purpose, which is a useful thing to remember when you are trying to fit him into a timeline.
Show detailsWhere this sits, and the version nobody counts
These films belong to the X-Men continuity, not the connected Marvel Cinematic Universe, which is why this catalogue files Wade Wilson under a different universe designation. That is a production fact rather than a story one, and the boundary has since been crossed in both directions.
Ryan Reynolds had already played the character once, in X-Men Origins: Wolverine in 2009, a version with his mouth sewn shut that the 2016 film disowns on screen. Wikipedia’s article on the series describes Reynolds in Deadpool as reprising that role, which is technically true and which the film itself spends a scene refusing to accept.
Minor spoiler
Vanessa is killed in the opening minutes by a target Wade failed to finish, and the film spends its first act on a man who repeatedly tries to die and cannot.