So Cain Marko's dad worked with Xavier's. Mr. Xavier died, Marko married Charles's mother,who herself died in turn. Cain was at boarding school for most of this . Years later the stepbrothers serve together in Korea and Cain's life has never been the same since.
Or maybe it's always been the same since. Whenever Juggernaut comes back into Charles Xavier's life, he sees Charles as the blonde kid he's smacking in the face. What he wants is nothing more or less than to drag Professor X kicking and screaming back to the past. Luckily, Charles has his X-Men, the manifest evidence that it is no longer the past, and that Xavier is no longer a frail boy, alone in the world.
Professor X left his empowered brother buried beneath a mountain and Korea. It took Juggernaut at least the better part of a decade (In Silver Age time) to dig himself out. It's understandable that he might be upset about that. His response is to attempt to drag Charles back to a state of childhood helplessness and presumably murder him. In any appearance of Juggernaut in an X-Men comic this is the most important dynamic at play. Juggernaut's assaults on the X-Men or on other things that represent Xavier's ideals are merely extensions of his long smoldering rage at his stepbrother. And what is the source of that rage? In depictions of their childhood it seems to be Xavier's very existence. The radiation that killed Charles's father is also implied to have triggered Professor X's mutation. When Charles's mother and Cain's father die too, they are left alone. Mutual orphan-hood didn't bring them closer together though, if anything being two beings connected by nothing but now deceased parents deepened Cain's resentment towards a brother he never asked for. The ways in which both men have changed by the time they meet again for the first time in the Silver Age complete their existences as warped reflections of each other. By that time Cain Marko had become the unstoppable Juggernaut a being of incalculable strength and near invulnerability, but also of unquenchable murderous rage. Charles Xavier has become Professor X, a man possessing the most powerful mind on Earth, but with a relatively frail body.
To the X-Men themselves, Juggernaut should represent the greatest test of their ability to function as a team in the field. If there's currently acrimony between team mates (When isn't there?) then it should come up as an issue during the fight. In light of the X-Men's focus on school and the future, it is often appropriate to view each X-Men rogue as embodying some broad lesson or test. Magneto, for example, may be seen as a lesson on letting oneself continue to be mired in past tragedies, regardless of the scope of that tragedy. Juggernaut, on the other hand, can be seen as a lesson on the ultimate limits of self sufficiency. No matter how strong or invincible he is, Cain Marko doesn't stand a chance once the X-Men get their collective shit together. This is even emphasized in the X-Men animated series of the nineties. It's depiction of the first X-Men/Juggernaut fight makes Colossus, a citizen of a then recently communist country, play a pivotal role in stopping Juggernaut's rampage. Of course, the team usually struggles a bit in the early stages of a fight with him.
Juggernaut doesn't just hang out beating up teenagers though. His strength and powers have made him a popular foil for quite a few non mutant heroes. In the late sixties his mystical origins (More about which in a moment.) led to a confrontations Dr. Strange.
He has also fought Spider-Man a number of times. These fights have the particularly fun element of Spidey's speed, agility and brains verses Juggernaut's strength, stamina, and brawn.
For similar reasons, indestructable man vs. man with healing factor stalemate, he makes an interesting solo enemy for Wolverine.
Juggernaut's non X-men appearances have an odd tendency to both round and flatten his character at the same time. They round it, in the sense that they show him capable of motives beyond 'kill my stepbrother and everything he believes in'. They flatten it in that said other motive is usually just 'do mercenary work for money' and 'walk through anything that dares get in my way'. Oddly enough, for a man so consumed by essentially self-centered issues relating to his childhood and proving himself superior to Charles Xavier, he actually has managed to make and maintain one significant friendship of sorts down through the years.
They were apparently drawn together out of a mutual loathing of relatives, as Black Tom Cassidy is primarily interested in revenge on his cousin Banshee. He's a mutant who's power is the ability to shoot energy blasts out of anything made of wood that he's touching. Their partnership has gone back and forth over the years. Here they are celebrating a small victory over the X-Men.
It's nice to have a friend. He also had a team up, of sorts, with She-Hulk at one point.
Try not to feel bad for Jen's dignity. It turns out that it was a version of her from an alternate universe.
At the time he was attempting to reform as a member of the X-Men. All villains who get popular enough, except maybe the J-man, tend to get a run at redemption and a spot on a super team, eventually. The same is true of Juggey here who was on the team during the often reviled Chuck Austin Uncanny X-Men run of the early aughts. As with Magneto, I think this is wrong headed, even more in this case, Magneto at least had ideals at one point, Juggernaut's really never done anything but brutalize and bully everybody who isn't Black Tom. Also like Magneto, it's a case where the established iconography of villainy and outright monstrosity is so powerful that trying to tame him really dilutes his power.
The most important visual element of Juggernaut's design is the helmet. It gives hims a smooth round head, like Xavier has naturally, and and also like Magneto has given himself artificially with his own helmet. Like Magneto's helmet, Juggernaut's speaks of closing off ones mind to the outside world. Also like Magneto's helmet, there is a connotation of a past world. However, where Magneto's helmet reminds one of the ancient warriors of classical Greece and Rome, Juggernaut's seems to have connotations leading one to the superstitions of the Dark and Middle Ages. The helmet, again like Magneto's, leaves Juggernaut one of the few beings on earth that Xavier's mind cannot effect. The helmet also functions like Magneto's in that it visually and symbolically dehumanizes the appearance of the villain. The helmets mark both of them as people who have chosen to divorce themselves from the human race, albeit in different ways and for different reasons.
There's a word pivotal to understanding Juggernaut's iconic power that I've been saving until now: Bondage. There is a heavy undercurrent of bondage and fetishism in the depiction of Juggernaut. It has to do with the sort of rude sexual nature of massively muscled male figures in western culture. Amy Poodle of the Mindless Ones writes about it in the Rogues Review focused on Bane. Given that his most important relationships are with a slightly fay Irish mutant of ill repute, and his bald slightly fay stepbrother, it is easy to make the jump and talk about Cain Marko in terms of the fetishistic and homoerotic. His relationship with his brother is driven by his desire to re-establish the old order and place Xavier under total domination, by killing him we assume. His powers are mystical in nature. This places him at odds with Xavier's gospel of scientific and social enlightenment, but also places Juggernaut in the service of an other worldly demon/god, Cyttorak.
Cyttorak appears in many incantations performed by various Marvel magic characters. When invoked, Cyttorak provides those who call upon him with the power of both containment and protection. The god Juggernaut owes his powers to is one of protective domination. Any Juggernaut story that isn't about Juggernaut trying to kill Charles Xavier or doing mercenary work tends to be about him performing tasks for the god that empowers him. The price for the power that Juggernaut wishes to use in domination of Xavier leaves him, in turn dominated by Cyttorak. The red bands on Juggernaut's arms are a reminder that, unlike a mutant, his powers are not his own. Then there's the design of his incarnation in the Ultimate marvel universe.
Looks a bit like another symbol of dominating male eroticism.
But even more-so it gives the impression of that icon of fetish wear, the gimp suit.
I'm not, of course saying that any of this is, or should be, overt, but there's no denying that his self-dehumanizing super-masculine desire for domination is a strong aspect of Juggernaut as a villain.
When I think of Juggernaut, I think of a chthonic force looking to drag an individual's progress back under the mountain. Juggernaut comes from the mountain and he is the mountain. He's a creature of extreme self-sufficiency, but relies on an outside power for that self-sufficiency. He's dark age magic in an age of scientific reason. He's the cruel relative writ large. He's a hurtling chunk of muscle waiting for you to sacrifice yourself beneath his tread. He's the red helmet battering down your walls. He's the earthiest of the X-Men's foes with a costume colored brown and red to match, and motives rooted in family life, looking mainly for revenge and creature comforts between looking for revenge. He's your past insisting that nothing's changed and you're as pitiful as ever. The X-Men is a fight for the future to exist the way it wants to exist. If you want to know Juggernaut's vision of the future imagine the title of this post - forever.