Juggernaut isn't the smartest of the X-Men villains, but he can be quite the nuisance given the fact that if he gets enough momentum going he is literally unstoppable. Juggernaut's greatest asset is his size and strength which he uses plenty of times. His power can also be amplified through a device. He wears a red helmet much like Magneto simply to keep his step brother Charles Xavier out of his head. His brutality is one of his best abilities, and if he didn't have a helmet then a telepathy of Xavier or Jean Grey's ability would be able to neutralize him fairly easily.
Cain Marko grew up in a volatile upbringing. When he was young his peaceful and loving mother died leaving him in the violent hands of his abusive father, Doctor Kurt Marko. With his father, Cain endured physical and psychological torment throughout his childhood. Following the death of Cain father's colleague Doctor Brian Xavier, Kurt married his widowed wife Sharon. Kurt and Cain then moved in to the Xavier estate with Sharon and her young son, Charles. Over the years, Kurt seemed to prefer Charles over his own son. This in fact, made Cain's jealousy of Charles grow which resulted in Cain frequently bullying Charles. Cain's father continued the beatings, but this time he was not alone. Charles was inexperienced with his mutant telepathic abilities and began to share the abuse that Cain was so frequently enduring. Charles felt the abuse, but also the immense amount of jealousy Cain had over Charles. Cain was jealous of his relationship with his mother, the relationship he had built with Kurt and the lifestyle he has grown up in. All of this coupled together to create an extremely unhappy and angry person that promised himself to always hate Charles Xavier. Eventually, Cain and Kurt had an argument which lead to Cain accidentally causing a fire to erupt in Kurt's home laboratory. It pained Cain to see his own father go out of his way to save Charles before his own son. Kurt did however save both children from the father, and due to smoke inhalation. Before dying, Kurt whispered to Charles to always beware of Cain.
As Cain and Charles grew up, they were eventually drafted into military service and just so happen to serve in the same unit. Cain eventually was deserted under fire during a mission he had in Asia, Charles pursued him hoping to persuade him to return of his own accord. Cain then found a cave that Charles followed him into. Inside the cave was the lost temple of an entity called Cyttorak. Cyttorak was a long lost powerful magic entity. While searching throughout the ruins eventually Cain discovered a mystical gem. Upon touching it, Cain was then transformed by it's magical powers into a walking and talking juggernaut. This transformation eventually lead to a cave-in which Charles was able to escape from. Cain survived the cave-in and came up from it with his new found powers and plans to use it to exact his revenge on his step brother.
In every one of his attempts to get his revenge on Charles, he was almost always bested by the X-Men. Upon getting beaten every time by Charles' group of X-Men, Cain decided to pair up with another mutant criminal like himself named Black Tom Cassidy. Quickly the pair became the world's most wanted criminals. After years of his continued villainy, Cain was teleported to an alternate universe. In this alternate universe he helped a group of costumed adventurers who called themselves the Exiles. As time passed, eventually Cain was transported back to his home planet only to be brought to the front door of death by one powerful entity named Onslaught. Onslaught threw Cain from Canada all the way to New York, in New York he encountered the X-Men who brought him back to the mansion for medical treatment. Upon waking up, he wanted to tell Charles the threat that Onslaught poses to everyone but upon viewing Charles' face he saw Onslaught once more. Cain found out that Charles took the gem from him and trapped it within himself where Cain then found an entirely new plane of existence on top of a corrupt aspect of Cyttorak. In this realm, he found a mystic by the name of Gomurr that sought out to help Cain free himself from the jewel and imbue him with it's power. Using his new found power, Cain then was able to destroy the corrupt aspect and free himself from the gem and in fact returned himself more power than ever before. Gomurr warned him that he was destined to destroy everything, but Cain ignored this warning and continued his ways of being a villain.