After a botched appearance in 2009's X-Men Origins: Wolverine, Deadpool is finally getting a solo film this weekend. As always with the X-Men movies, characters are mixed and matched; mutants who have never appeared in Deadpool comics appear because it's fun fan service. But for the most part, the translation from the page to the screen is flawless. Here's a [warning: somewhat spoilery] primer on the characters that pop up in Deadpool and how they connect to the Marvel Comics universe at large.
Deadpool, a.k.a. Wade Wilson, earned his superpowers after being a part of the Weapon X program — a government genetic research initiative that turns subjects (sometimes against their will) into living weapons. The program, which also created Wolverine, is a descendant of the Weapon I program that produced Super Soldiers (most notably Captain America) during World War II. Wade joins the program in an attempt to cure his cancer, but is horribly scarred in the process and given the ability to regenerate after being wounded. A wisecracking mercenary, Deadpool often takes the piss out of other Marvel characters and breaks the fourth wall, speaking with the audience. In the translation to the film version, Deadpool's origin has remained pretty much intact. In the film, however, the program isn't referred to as Weapon X, it's just a generic clandestine research project. This means that the Deadpool who appeared in X-Men Origins: Wolverine is not the Deadpool now in the Marvel Cinematic Universe canon (despite still being portrayed by Ryan Reynolds).
But as far as comic-to-film translations go, Deadpool remains the lovable, foul-mouthed asshole we know and love.
Vanessa Carlysle (a.k.a. Copycat)
Deadpool's girlfriend is a prostitute who moonlights as a stripper. (Or maybe it's the other way around.) Either way, he "rescues" her from the life only to abandon her when he contracts cancer so she doesn't have to see him die. In the film, she's only a human (played by Morena Baccarin). But in the comics, she's the shape-shifting mutant known as Copycat. She has been a member of New Mutants (along with Deadpool), as well as X-Force, where she went undercover as the hero Domino in order to kill Cable (the son of the X-Men's Cyclops and a clone of Jean Grey, Madelyne Pryor … but let's not get into X-Men history here; it'll take all goddamn day).
Deadpool's best friend and bartender, Weasel (played by Silicon Valley's T.J. Miller), is pretty much the comic relief in the film. In the comics, his existence is somewhat more tragic. He attended high school with Peter Parker (Spider-Man) and Jessica Jones, and was up for the same job with Norman Osborn that Peter was. Deadpool, however, transported back in time and told Norman that Weasel was a drug user and ruined his chances at the job. He turned to a life of crime and in turn has become not only Deadpool's friend but his arms dealer.
Blind Al's existence is much darker in the comics. Not only is she Deadpool's houseguest, she's his prisoner, and he also plays cruel pranks on her. In the film, she's much more of a sidekick and lives with him thanks to the powers of Craiglist. The cruelest thing she has to endure is listening to Deadpool masturbate loudly in their apartment; fortunately, she's unable to see it because she's blind. Pour one out for Al.
Deadpool's nemesis in the film, Ajax (a.k.a. Francis), is the man who operates on Wade and gives him his powers. In the comics, he's merely the enforcer of a man named Dr. Killebrew. This makes much more sense than the film's version, because Ed Skrein is hot as fuck and his casting as a super-genius evil scientist is like Denise Richards playing Dr. Christmas Jones in The World Is Not Enough.
Colossus, a Russian member of the X-Men who can turn himself into organic steel, isn't a fixture in Deadpool's life outside the film. Deadpool doesn't have much interaction with Charles Xavier's team (he is a member of X-Force, however) except for in the comic Deadpool Kills the Marvel Universe, when they send him to a psych ward to get therapy.
Negasonic Teenage Warhead
Deadpool cracks a joke in the film about only ever seeing Colossus and Negasonic Teenage Warhead when he shows up to the X-Men's mansion, because Fox couldn't afford any other X-Men actors -- which rings pretty true when you're stooping to include Negasonic Teenage Warhead in your movie. In the film, she's actually a pretty dope character who can project bursts of energy, much like a warhead. But in the comic books, she's a student of Emma Frost (portrayed by January Jones in X-Men: First Class) and a telepath. She was dubbed Negasonic Teenage Warhead from the song by the band Monster Magnet.
Oddly enough, Colossus's girlfriend in the comics, Kitty Pryde, once said of Negasonic, "Wow, we really have run out of names."
Ajax's henchman in the film has no prior interaction with Deadpool in the comics. She appears in the Marvel comics universe -- she's a member of the Morlocks, a subterranean race of mutants. She ran away from home as a young mutant to join them. In the comics, the X-Men's Storm often has interactions with the Morlocks.