Okay, the secrets out – as a kid, I was the biggest X-Men fan in the world. I only know the Avengers through their occasional crossovers with the X-Men. There was a time where I could recite every character that was ever officially or unofficially in the X-Men or a spin-off team.
That’s a lot of useless knowledge.
Instead of exploring Earth-199999, let’s take a break from the Marvel Cinematic Universe and look at Deadpool supporting characters over in the-earth-that-was-Earth-10005.

Source: totalcomicmayhem.com
So, first up – let’s get the hard part out of the way. As much as we try to lie to ourselves, Deadpool himself has been in the X-Men Cinematic Universe before – he was even played by Ryan Reynolds and everything. In the undisputed weak link of the XCU, X-Men Origins: Wolverine, we had Wade Wilson as one of the soldiers/experiments with Wolverine in Project X. Except, after the experiments, he was hardly the Merc with the Mouth since he didn’t have a mouth. He also had claws and laser eyes and – WHY? Luckily, all of that was reconnected when they changed the timeline by altering the past in X-Men: Days of Future Past.

source: moviepilot.com
Now let’s move on to the supporting cast of Deadpool’s upcoming movie – we can meet them before we meet them. Though he’s no longer played by Daniel Cudmore, Colossus is making his return to the XCU. Colossus is Piotr Rasputin, a Russian man with the mutant ability to turn his skin into metal. He appeared in the original trilogy, before returning and dying in X-Men: Days of Future Past. Of course, the timeline reset – so now he’s alive again (and appears to be actually Russian this time around. Three cheers for reconnecting!)

Source: Nerdist
Also appearing in Deadpool is his trusty sidekick, Weasel. Jack Hammer (ha, ha) is the geeky guy who provides Wade information, weapons, and fixes his equipment. He’s probably the closest thing Deadpool has to a friend, despite him treating him pretty terribly.

Source: tilly-stratford.livejournal.com
Blind Al has got to be one of the strangest characters in the Deadpool comics. An elderly lady named Althea (though Wade calls her Albert), Deadpool has kidnapped her and now she serves as a mix between his prisoner, roommate, and mother figure. She is cynical, wise-cracking and aggressive, and doesn’t seem to mind the endless pranks and ways Deadpool takes advantage of her blindness.

Source: Polygon.com
In a movie, another member of the Weapon X program is Ajax. In the comics, there have been two characters with this name; the first was a Hulk villain and descendant of the demigod Agamemnon, while the second one (and most likely to be the one the movie is based on) is a super-strong mutant who was part of the Weapon X program, and has dedicated himself to hunting down Deadpool and his loved ones for seemingly killing him years earlier.

Source: moviepilot.com
We will end this edition with former X-Force infiltrator Copycat. Vanessa Carlysle is a mutant with the ability to shapeshift and even copy other mutant abilities. Before Wade got cancer and became Deadpool, Vanessa and Wade dated, him saving her from a life of prostitution. Vanessa later became a mercenary herself, and was tasked to pose as Domino, infiltrate the X-Force and kill Cable – before she was outed by her former lover. While Vanessa is in the movie as Wade’s girlfriend, it is unknown if she will exhibit any powers.

Source: Marvel Comics
While there are many more characters from the comics and we could do this all day, we will end here. Save something for the silver screen, right?