Dedicated to protecting a world that often hates and fears them, the X-Men are Marvel's premiere mutant hero team. Whether operating as a school, a strike force, or an independent island nation, the X-Men's ranks have been primarily comprised of carriers of the power granting X-gene, colloquially known as "mutants."
While Marvel's mutants have always been at the center of each incarnation of the X-Men, the team's allies have not always been mutants themselves. In fact, there have been a number of non-mutant supporting charters in X-Men comics throughout the years, with some even gaining official team membership.
Valerie Cooper
Debuting in Uncanny X-Men #176, Valerie Cooper has been portrayed as both an antagonist and an ally to the X-Men and their associates. When she was first introduced, it was as a US National Security Advisor assigned to oversee Mystique's version of the Brotherhood. Despite boasting some of the Brotherhood's most powerful members, Mystique's team had accepted a government contract in lieu of prison.
When that team inevitably imploded, Cooper's next assignment was as liaison to the second version of X-Factor, a joint operation featuring former X-Men and their associates working for the NSA. With X-Factor, Cooper received her most prominent role to date and often fought alongside the team on missions. Although her loyalties were always to her country first, it's clear she also believed in Xavier's dream of human and mutant coexistence.
Peter Corbeau
Doctor Peter Corbeau is a certified genius, scientist, and astronaut; the college roommate of Bruce Banner, and a longtime friend of Charles Xavier. After first appearing in issues of the Incredible Hulk, he transitioned over to the X-Men comics following a crossover. Corbeau played a prominent role in one of the greatest X-Men stories of all time when he joined them on an adventure into space, during the return trip from which Jean Grey would be replaced by the immortal Phoenix Force.
He returned shortly after in the acclaimed follow-up The Dark Phoenix Saga but has made rare appearances since. Despite this, he has recently gained a new fandom thanks to the podcast Jay and Miles X-Plain the X-Men, in which he is referred to as "Super Doctor Astronaut Peter Corbeau" and receives his own jaunty sailor theme whenever he is discussed.
Captain Britain
Betsy "Psylocke" Braddock has been a fan favorite member of the X-Men for years. Casual fans may not be aware though that her brother is an accomplished superhero in his own right: England's premier adventurer Captain Britain.
While Brian Braddock wasn't born with any mutant abilities like his famous sibling, he was granted mystical powers stemming from Arthurian Legend and as Captain Britain, he has proven to be just as valiant a hero. Despite his lack of X-gene, Brian is a founding member of the European mutant team Excalibur and a frequent X-Men ally.
Abigail Brand
Introduced in Joss Whedon's critically acclaimed Astonishing X-Men, Abigail Brand is the commanding officer of S.W.O.R.D., the Sentient Worlds Observation and Response Department. Brand and her organization are complementary to S.H.I.E.L.D. but with a mission statement centered on potential threats originating from off-world.
Despite seeing mutants as a bit of a nuisance for their regular involvement in interplanetary matters under her jurisdiction, Brand has gained gradual respect towards the X-Men and teamed up with them on a handful of occasions. She once even briefly dated Beast, one of the most prominent X-Men and a member of Xavier's first class of students.
Fantomex
The Weapon X Program that gave Wolverine his unbreakable skeleton has become legendary in X-Men comics, but Grant Morrison's early 2000's tenure as the architect of the mutant mythos added new layers to this classic tale. As revealed in New X-Men #128 (2002), Weapon X was actually an offshoot of the Weapon Plus Program: the joint US and Canadian government project to create super soldiers in the style of Captain America, whom they retroactively labeled "Weapon I".
While comic fans know Wolverine (as Weapon X) was involved in the tenth incarnation of the project, further experiments were performed after his creation. Fantomex, also known as "Weapon XIII" owes his existence to this clandestine organization. A "super-sentinel" created from fusing organic and technological material, Fantomex eventually broke free from his creators and joined the X-Men full time. His chosen human persona is an exaggerated interpretation of a French super-spy, complete with requisite cliché speech patterns and mannerisms.
Danger
Unbeknownst to the X-Men, the integration of alien Shi'ar technology to upgrade their training facility, the Danger Room resulted in a burgeoning artificial intelligence that spent years trapped in an incorporeal form. When she finally gained the ability to physically manifest herself, this living embodiment of the Danger Room, aptly named "Danger" immediately became an X-Men enemy, before being subdued and eventually realigned to the mutant cause.
A fully self-aware artificial construct, Danger has nonetheless identified with the persecution of mutants and aligned herself with the X-Men, frequently serving as an active field combatant as well as technological and strategical support, bolstered by her years of studying the methods of Xavier's mutants while in her previous incorporeal form.
Lilandra
Occasional Empress of the alien Shi'ar Empire, Lilandra Neramani is also an on and off love interest of X-Men founder Charles Xavier. In addition to her duties as the monarch of a multi-planetary empire, Lilandra has also featured as a prominent recurring ally to the X-Men throughout the years.
Lilandra has been present during many of the most notable mutant space adventures and chaperoned the Professor during his own sabbatical from the earth following the Brood Saga. Long-term fans will also remember her recurring role in the 1992 X-Men Animated Series which faithfully adapted her role as a supporting character in some of the X-Men's most famous stories.
Spider-Man
Despite Marvel's mutants often existing in their own corner apart from the greater Marvel Universe, Spider-Man has built some strange friendships with the X-Men's members and has fought alongside the mutant team in many notable adventures. After decades of team-ups, the web slinger's most notable tenure as an X-Men supporting character came shortly after 2011's Schism event when Wolverine restarted the classic mutant school under his own design. Refusing to remove his mask, even amongst the telepaths present, Peter Parker utilized his past experiences teaching students at Midtown High School as the instructor of the newly renamed Jean Grey Acadamy's "special class."
While he joined as a favor to his longtime ally Wolverine, Spider-Man, despite all of his credentials, struggled in this role. Regardless, his adventures with the newest class of mutant students were memorable, and his time as "Professor Spider-Man" during Jason Aaron's Wolverine and the X-Men represents the culmination of years of history in which Spider-Man and the X-Men were Marvel's most important properties.
Longshot
Possibly the most prominent non-mutant to ever attain official X-Men membership, Longshot was a featured part of the core X-Men ensemble during the fan-favorite 1980s "Outback Era". Not only is Longshot not a mutant, he is additionally not even human, but rather an artificially created humanoid being designed to entertain the spineless masses of the Mojoverse.
Despite Longshot's lack of an X-gene, he was created with a set of superpowers of his own, including incredible dexterity and a "good luck" power that causes events to frequently fall in his favor so long as his intentions are pure. After leaving the X-Men as the 90s rolled in, he was returned to a central role in Peter David's X-Factor vol. 3, in which his origins became even more complicated.
Lockheed
When Lockheed first joined the X-Men during the Brood Saga, the diminutive alien dragon was depicted as having the approximate intelligence of a dog. After bonding with the team's youngest member, Kitty Pryde, he would accompany her unwaveringly, even joining her as a founding member of Excalibur. Gradually, however, it became apparent that Lockheed was smarter than he let on.
Eventually, it was revealed that Lockheed was in fact highly intelligent, and despite his generally preferred silence, capable of speaking numerous human and alien languages. One of the most unassuming of the X-Men's allies, he has saved the team on multiple occasions. Not bad for a tiny purple dragon.