Professor Charles Xavier is the founder and mentor of the X-Men in the Marvel Comics multiverse. A wise, compassionate and powerful mutant psychic, Xavier (or Professor X as his students often call him) dreams of a world where humans and mutants can co-exist in harmony. In the meantime, he has established the Xavier School for Gifted Youngsters as a safe haven for young mutants who are struggling against society's prejudices and commands the X-Men to battle against those who would threaten his vision of human/mutant cooperation.
Professor X is portrayed by Sir Patrick Stewart in the original X-Men film trilogy and by James McAvoy in the reboot series beginning with X-Men: First Class.
In X-Men 3: The Last Stand, both Xavier and his long-time friend and rival Magneto visit the former residence of Jean Grey when she was a child. Having recently returned from the dead, Jean's telekinetic powers are growing beyond her control as her dark personality - the Phoenix - resurfaces after years of dormancy. Charles wishes to help Jean bring her abilities under control and prevent the Phoenix from lashing out and harming others, but Magneto sees an opportunity to win Jean over to his cause of mutant supremacy, contradicting Charles' words and urging Jean to set the Phoenix free.
Although Magneto does convince Jean to join his Brotherhood, it comes at a terrible price. The fury of the Phoenix is unleashed as Jean's psychokinetic power begins slowly dissolving her surroundings at the atomic level. Focusing her rage on Xavier, Jean holds her mentor suspended in mid-air and then annihilates him, completely disintegrating his body.
Although Xavier's body was destroyed, he is not entirely vanquished. In a post-credits scene, one of Xavier's old colleagues, Dr. Moira MacTaggert, is monitoring a patient of hers: a mutant that appears comatose, but in fact, has no mind to speak of despite being physically alive. Whilst Moira is standing over her patient's bedside, the mutant suddenly greets her in Charles Xavier's voice, indicating that Xavier's mind has now occupied the vacant body.
In the dystopian future of X-Men: Days of Future Past, Xavier - alongside his old friend and former adversary Magneto - leads a small band of mutants in a losing war against the Sentinels: deadly machines capable of assimilating the powers of mutants which have all but exterminated the world's mutant population. With the aid of Kitty Pryde's chronoskimming ability, they are able to project Wolverine's consciousness back through time to 1973, allowing him to make contact with the young Charles Xavier (portrayed by McAvoy) and enlist his help in altering events so that the Sentinels never take over. Thanks to Wolverine, Xavier is able to convince Mystique not to carry out the assassination of Bolivar Trask, inventor of the Sentinels. Trask is later arrested and stands trial for his experiments on mutants and the Sentinel Program is scrapped, thereby altering the timeline. Not only is the Sentinels' future erased but the events of the original X-Men films do not occur either.
In the 2017 film Logan, Xavier's life comes to a violent end. By 2029, Xavier is one of the last mutants left on the planet. In his old age he has developed a form of dementia not unlike Alzheimer's. It was particularly dangerous for Charles as he could no longer control his immense psychic powers. He began to have massive uncontrollable psychic seizures, one of these seizures took place while he was in the X-Mansion. It effected over 600 people in Westchester and resulted in countless injuries and the death of several members of the X-Men.
Soon after Wolverine removed him from society, arranging for him to be cared for in an abandoned smelting factory so that his psychic episodes could not hurt anyone else. While there, Xavier began to sense a foreign mutant, who was later discovered to be Laura Kinney, Wolverine's daughter.
After Transigen came after them, Wolverine, Xavier, and Laura went on the run, heading towards a supposed mutant paradise called Eden. While on the way Xavier endured another psychic seizure in a casino, nearly killing hundreds of people in the hotel and buildings around before he was sedated by Wolverine.
While resting at the home of a family they had assisted, Xavier began to remember the circumstances of his isolation, and how he had been the reason many of his students were paralyzed or dead. He believed himself unworthy of Wolverine's help, declaring this to X-24, whom he believed to be Wolverine. X-24 stabbed him through the chest, a wound which would prove fatal.
Wolverine buried him in an unmarked grave in a forest beside a lake.