Keep your friends close and your enemies even closer. That old saying seems to be the motto of the X-Men, because they've been willing to take in anyone as long as he or she at least tries to be reformed. That fits into Professor Charles Xavier's message of peace and harmony with all people, especially between mutants and humans, but also believing in the good of others.

RELATED: Worst X-Men Ever

While the X-Men have faced some of the most deadly supervillains in the Marvel Universe, they've also made some of their worst enemies into honorary members of the team. Sometimes, that's worked out great and the former villains became loyal X-Men. Other times, the villains have gone right back to their evil ways. Here's a run-down of some of the worst villains who joined the X-Men.

15 TOAD

Marvel Comics' Toad crouched down and grimacing in Marvel Comics

The man who would become Toad started out in "The X-Men" #4 in 1964 as an enemy of the mutant superheroes, created by Stan Lee and Jack Kirby. He was a mutant named Mortimer Toynbee with a hunchback and a knack for jumping who joined Magneto's Brotherhood of Evil Mutants, playing the role of Magneto's sidekick for years until he broke away to form his own Brotherhood of Evil Mutants.

Toad had been a villain for most of his history until 2011's "X-Men: Regenesis" #1, written by Kieron Gillen and penciled by Billy Tan. At that point in the saga of the X-Men, the team had broken up into two factions; one led by Cyclops and the other led by Wolverine. Wolverine took over the X-Mansion and renamed it "The Jean Grey School For Higher Learning." In issue #1 of "Regenesis," Toad begged to join Wolverine's team, even as a lowly janitor. It wasn't the best role in the X-Men, which may be why he quit to re-join Magneto soon after.

14 MIMIC

mimic-x-men

When Mimic made his first appearance in 1966’s “X-Men” #19 (artist Werner Roth and written by Stan Lee), Calvin Rankin was caught in a chemical accident by his father which gave him the ability to take on the knowledge and skills of anyone he wanted. When it came to the X-Men, Mimic discovered he could absorb the Beast’s athletic ability, Cyclops’ power beams, the freezing powers of Iceman and Marvel Girl’s telekinesis, but Professor X erased his memory.

In 1966's "X-Men" #27 (Roy Thomas, Werner Roth), Mimic was caught in a blast that revived his memory, and he demanded to be allowed to join the X-Men. With his combination of all their powers, along with Professor X's mental abilities, the X-Men seemed helpless in front of him. It turned out the X-Men were just pretending and went on to beat him, but Mimic came back to become an enemy and an ally many times, and even joined Norman Osborn's Dark X-Men team. He could copy their powers, but not their honor.

13 DEADPOOL

deadpool-rob-liefeld

He's a beloved antihero now, but the "merc with a mouth" started out as a supervillain in "The New Mutants" #98, created by Rob Liefeld and Fabian Nicieza in 1991. Deadpool was a sarcastic killer-for-hire who fought for different supervillains like the Kingpin, and sliced his way through the New Mutants and later X-Force. Over time, he evolved into a genuine hero who got his own miniseries. He loves guns and swords and little pouches, and people keep letting him onto their teams for some reason.

In 2004's "Cable & Deadpool" #8 (Fabian, Nicieza, Patrick Zercher), Deadpool is actually hired by the X-Men to stop Cable, who's launched a floating fortress in a desperate plan to "save the world." The X-Men reluctantly agreed to bring Deadpool onto the team, even though he demanded to wear Jean Grey's uniform. Deadpool ended up moving on from there, but the X-Men never forgot him, especially how he looked in a thong.

12 QUICKSILVER

Quicksilver

As far as teams, Quicksilver is known more for hanging around with the Avengers, but he's also been a part of the X-Men, even though he started out as an enemy. With his superhuman speed, he was first introduced in 1964's "X-Men" #4 by Stan Lee and Jack Kirby as a member of the Brotherhood of Evil Mutants, along with his sister Scarlet Witch, Toad, Mastermind and Magneto. He fought Xavier's attempts to work with humanity, especially when he discovered he was the son of Magneto. Quicksilver went on to join the Avengers for many years, but didn't stop there.

In "Uncanny X-Men" #304 (1993) by Scott Lobdell and Jae Lee, Quicksilver first crossed over to the side of the X-Men. In the story, Magneto returned for his revenge against the X-Men by bringing his fortress Avalon to Earth, bristling with alien weaponry. Quicksilver refused to join his father, siding with the X-Men instead. From there, Quicksilver would leave and join the X-Men, the Avengers, and manipulated Scarlet Witch into altering the universe in the "House of M" storyline, always following his own moral code.

11 CALLISTO

Callisto

In 1983's "Uncanny X-Men" #169, Chris Claremont and Paul Smith created Callisto, the mutant leader of an underground gang of mutants. Callisto turned a Cold War bomb shelter into a hidden home beneath New York City, where mutants who were outcasts among humans could find peace. Many of the Morlocks were disfigured, but she wasn't above disfiguring them herself if they weren't when they arrived. She also kidnapped surface dwellers like the winged X-Man Angel. When the X-Men came after him, Callisto fought Storm for control of the Morlocks and lost, leaving herself humiliated and forced to submit to the X-Men.

In 1986's "Uncanny X-Men" #211 (Chris Claremont, John Romita Jr. and Bret Blevins), the ruthless cyborg mutant hunters known as the Marauders slaughtered most of the Morlocks. Callisto was one of the only survivors, and was forced to join the X-Men to escape, survive and heal. From there, she became more of an ally of the X-Men instead of an enemy.

10 DANGER

Danger Robot of the X-Men in Marvel Comics

The Danger Room has been a part of the X-Men since "X-Men" #1 (1963), created by Stan Lee and Jack Kirby. As a room in the X-Mansion that the mutant heroes could train in, at first it just contained a lot of traps (like trapdoors and moving walls) and weapons (like cannons and buzzsaws), but the Professor X upgraded it with alien Shi-ar weapons to make holograms of different environments for more realistic practice and combat.

Up until 2005's "Astonishing X-Men" #7 (by Joss Whedon and John Cassaday), the Danger Room was just a room, but that's the issue where the Danger Room revealed it had developed its own thoughts and personality. It turned out the Shi'ar technology took on the form of a robot woman called Danger. She wreaked havoc on the X-Men until 2009 in "Uncanny X-Men" #515 (Matt Fraction and Greg Land), when she decided to join the X-Men to study humanity.

9 SABRETOOTH

Sabretooth from X-Men

Sabretooth was originally created by Chris Claremont and John Byrne as a vicious enemy in "Iron Fist" #14 (1977), but the maned supervillain quickly became Wolverine’s archenemy whose idea of fun is killing innocent people. Sabretooth is as strong, fast and has the same rapid healing as Wolverine, but without a conscience and brings along a sadistic love of torture and murder. He's been the bane (pun intended) of Wolverine's life for decades, and seems like the last person who would join the X-Men, but it happened.

In 2006's "X-Men" #192 (Mike Carey, Chris Bachalo), Sabretooth went to the X-Men for sanctuary, because he was being chased by a group called the Children of the Vault. In order to get him to join their team, Rogue injected Sabretooth with nano-sentinels that they could activate to kill him on command in hopes of putting him under control. It didn't last long, and Sabretooth escaped, but it was a rare moment where he fought by their side during his evil days. In the recent "AXIS" storyline, Sabretooth's personality was inverted, making him as good a hero as he was a villain.

8 MYSTIQUE

Greg land mystique

First introduced in "Ms. Marvel" #16 by Chris Claremont and Dave Cockrum in 1978, Mystique is a mutant with the power to change her body and voice to match anyone she wants, perfectly. That makes her a master of disguise, and she's worked for decades to further the cause of mutants or just help herself. She's one of the deadliest enemies of the X-Men, but she has deep ties to them as well. She was revealed to be the mother of Nightcrawler and foster mother of Rogue.

In 2005's "X-Men" #171 (Peter Milligan, Salvador Larroca), Mystique joined the X-Men in the form of a girl named Foxx, but was caught by Emma Frost's mind reading. Even though she fooled them, the X-Men voted her in as a probationary member, but asked her to leave anyway. She returned in 2006's "X-Men" #181 (Peter Milligan and Roger Cruz), but ultimately turned on the X-Men to side with the Marauders. Double-crossing is her thing.

7 JUGGERNAUT

juggernaut-x-men

Nothing can stop the Juggernaut, the supervillain who first appeared in “X-Men” #12 in 1965, created by Stan Lee and Jack Kirby. Professor Xavier's half-brother Cain Marko stumbled into a cave during wartime and found the mystical Crimson Gem of Cyttorak. After coming in contact with it, the gem transformed him into the unstoppable Juggernaut. Besides incredible strength, Marko got the power to charge through walls and over his enemies and doesn't need to eat, sleep or breathe. The only thing that can stop him are psychic attacks. He's been a constant enemy of the X-Men, but occasionally he's worked for them, too.

In 2002's "Uncanny X-Men" #410 (Chuck Austen and Ron Garney), Juggernaut called the X-Men for help to escape from Black Tom Cassidy. Afterwards, he joined the X-Men with a secret plan to destroy the team, but when Cassidy betrayed him, Juggernaut tried to become a real X-Man and put his criminal past behind him.

6 BANSHEE

X-Men's Banshee in his original costume, flying in the air.

1967’s “X-Men” #28, created by Roy Thomas and Werner Roth, first introduced the Irish mutant named Banshee. Banshee (whose real name was Sean Cassidy) was named after the mythological Irish ghoul who would shriek before someone died, a fitting name since his power was to make screams so loud that they could shatter eardrums or allow him to fly. In his first appearance, Banshee was a villain, although reluctantly so. A gang named Factor Three put an explosive headband on him, threatening to kill him if he didn't work for them. Professor Xavier was able to remove the headband, freeing him to escape.

Later, in "Giant Size X-Men" #1 (1975) by Len Wein and Dave Cockrum, Banshee was recruited to join a new international team of the mutant heroes. He eventually became one of the most loyal members of the X-Men, even with his habit of losing his powers. He's no longer with the team, having sacrificed himself in battle, but he more than redeemed himself.

5 NAMOR

namor-the-submariner

Namor existed long before the X-Men, first appearing in 1939's "Marvel Comics" #1 by Bill Everett, back when Marvel Comics was Timely Comics. As the son of a princess from the underwater city of Atlantis and a human sea captain, Namor was gifted with superhuman strength, telepathic control over sea life and powers of flight. For most of his history, Namor has been an anti-hero in the truest sense, attacking the human world and wreaking havoc for the land-dweller's crimes against the ocean.

Despite his anger, Namor has been a team player, part of the Defenders and the secret Illuminati with Dr. Strange, Iron Man, Professor X, Black Bolt and Mister Fantastic. In 2009's "Uncanny X-Men" #513, Matt Fraction and Terry Dodson introduced Namor as one of the evil Dark X-Men, a team formed by Norman Osborn that he could control. During a climactic battle, Namor turned out to be working for the good guys, ending Osborn's reign of terror.

4 ROGUE

Rogue Bachalo

Rogue has been one of the most popular members of the X-Men since her first appearance in 1981's "Avengers Annual" #10 (Chris Claremont, Michael Golden). Her mutant ability to absorb the powers of others, along with her sassy and Southern ways, make her fun to read about with a touch of tragedy. She's also been loyal and stuck through the team through its adventures for the last 30 years. That all kind of overshadows the fact that she started out as a villain.

In "Avengers Annual," Rogue was a member of the Brotherhood of Evil Mutants, along with Mystique, Destiny, Blob, Pyro and Avalanche. In the issue, Rogue had erased Ms. Marvel's powers and memories, and used them to fight the Avengers, but eventually turned to the X-Men to help control her psychic trauma from absorbing people's memories. Xavier made her a probationary X-Men, despite the wishes of the other students, but she proved herself a true X-Man.

3 EMMA FROST

white queen

Now we turn to another longtime enemy, Emma Frost. Also known as the White Queen, Frost first appeared in 1980's "Uncanny X-Men" #129, created by Chris Claremont and John Byrne. Frost started out as a powerful telepath within the Hellfire Club, a secret society that claims to be a wealthy social club, but really tries to control world events for its own benefit. With her Black King Sebastian Shaw, Frost worked with and against the X-Men for years.

In Grant Morrison and Frank Quitely's "New X-Men" #116 (2001), the X-Men went to the nation of Genosha, where the psychotic Cassandra Nova had wiped out most of the mutant population. There, they found Frost, who had survived by changing to her secondary diamond form. At first, she refused to join the X-Men, but later changed her mind and became a controversial member and even a leader until she fled after "Inhumans vs. X-Men."

2 WOLVERINE

He's the best at what he does, but what he does isn't very nice. That's the famous catch-phrase of Wolverine, who made his first appearance in Len Wein and Herb Trimpe’s “Incredible Hulk” #181 (1974). He's become almost the face and mascot of the X-Men, their most popular member, and one of the most popular characters in the Marvel Universe. With his razor-sharp claws, unbreakable skeleton and rapid healing, Wolverine is a powerhouse for justice. Let's not forget, though, he started as a villain.

In his first issue, Wolverine was a super-powered assassin sent by the Canadian government to stop the Hulk and the white beast Wendigo. Logan didn't become a hero until "Giant-Size X-Men" #1 (1975), when Xavier called to bring him into the X-Men to rescue the original team. Since then, it's been established that Wolverine has been slicing and dicing his way through history, and continues to be a blood-drenched force for justice.

1 MAGNETO

How can someone be the greatest enemy of the X-Men and their greatest friend? When that someone is Magneto, the mutant who can control all metals. Created in 1963’s “Uncanny X-Men” #1 by Stan Lee and Jack Kirby, Magneto is the archenemy of the X-Men, leader of the Brotherhood of Evil Mutants. He uses his power to try to wipe out and enslave the human race, at one point trying to launch all the world's nukes to trigger World War III. He also attacked the X-Men directly to keep them from stopping his plans, and caused the deaths of thousands either directly or indirectly.

Yet Magneto shares the goals of the X-Men, even if his methods are different. In 1985's "Uncanny X-Men" #200 (Chris Claremont, John Romita, Jr.), Magneto was put on trial for his crimes but ended up saving Professor Xavier during an attack. Magneto reluctantly agreed to lead the X-Men and the New Mutants in Xavier's absence, but his manipulation and ruthless attitude caused them to rebel against him. Still, he's remained on the fence, working with and against the X-Men ever since.

Which supervillain were you most glad to see on the team? Let us know in the comments!