Unbreakable

In Philadelphia in 1961, Elijah Price (Samuel L. Jackson) is born with Type I osteogenesis imperfecta, a rare disease that renders sufferers' bones extremely fragile and prone to fracture. As revealed later in flashbacks, Elijahâwho grows up to become a comic-book art dealerâdevelops a theory, based on the comics he has read during his many hospital stays, that if he represents extreme human frailty, there must be someone "unbreakable" at the opposite extreme.
Years later, another Philadelphia man, security guard David Dunn (Bruce Willis), is also searching for meaning in his life. He had given up a promising football career during his collegiate days to marry Audrey (Robin Wright) after they were involved in an auto accident. Now, however, their marriage is dissolving, to the distress of their young son Joseph (Spencer Treat Clark).
As he returns home from a job interview in New York City, David's train crashes, killing the other 131 passengers, while he is the only survivor, sustaining no injuries. At the memorial for the crash's victims, he finds a card on his car's windshield, inviting him to Elijah's store. Elijah proposes to David that he is the kind of person after whom comic-book superheroes are modeled, and repeatedly pursues the issue with David and Audrey, trying to learn if David had ever been ill or injured during childhood. Although Elijah unsettles him, David begins to test himself. While lifting weights with Joseph, he bench presses 350 pounds, well above what he had thought he could do. Joseph begins to idolize his father and believes he is a superhero, although David still maintains he is "an ordinary man."
David challenges Elijah with an incident from his childhood when he almost drowned. Elijah suggests that the incident highlights the common comic trope whereby superheroes often have one weakness; he contends David's might be water. While surveying the stored wreckage of the train crash that he survived, David recalls the car accident that ended his athletics career, remembering that he was unharmed and ripped a door off the car in order to save Audrey. David used the accident as an excuse to quit football, because Audrey did not like the violence of the sport.
Under Elijah's influence, David develops what he thought was an unusual insight into human behavior into an extrasensory perception that enables him to glimpse criminal acts committed by the people who make contact with him. At Elijah's suggestion, David stands in the middle of a crowd in Philadelphia's 30th Street Station. As various people bump into him, he senses the crimes they perpetrated, such as theft and rape, and finds one he can act on: a sadistic janitor who invaded a family home, killed the parents, and is holding the children captive.
David follows the janitor to the victims' house and frees the children, but the janitor ambushes him and pushes him off a balcony into a swimming pool. David nearly drowns (since he cannot swim), but the children rescue him. He then attacks the janitor from behind and strangles him to death while once more remaining uninjured. That night, he and Audrey reconcile. The following morning, he secretly shows a newspaper article on the anonymous heroic act, featuring a sketch of David in his rain poncho, to his son, who recognizes the hero as his father.
David attends an exhibition at Elijah's comic book art gallery and meets Elijah's mother (Charlayne Woodard), who explains the difference between villains who fight heroes with physical strength versus those who use their intelligence. Elijah brings David to the back room of his studio, extends his hand, and asks David to shake it. Upon doing so, David sees visions of Elijah orchestrating several terrorist disasters, including David's recent train accident, causing hundreds of deaths. David is horrified, but Elijah insists the deaths were justified as a means to find him. Calling himself "Mr. Glass" (a nickname his peers had used to taunt him with when he was growing up), he explains that his own purpose in life is to be the villain to David's hero.
Screen captions reveal that David reported Elijah's actions to the police, and that Elijah was convicted of murder and terrorism and committed to an institution for the criminally insane.

Question: Who challenges Elijah?
Answer: David.