Sylar (Gabriel Gray) is a fictional character and a primary antagonist of the NBC science fiction drama series Heroes. Portrayed by Zachary Quinto, he is a superpowered serial killer who targets other superhumans in order to steal their powers. He served as the primary villain of the first season and then as a recurring antagonist. Over the course of seasons two, three and four, the writers depicted Sylar attempting at various times and under different circumstances to become a hero or curb his desire to kill, undergoing relapses at various stages.
Haunting Matt Parkman
Meanwhile, Matt Parkman, regretting his actions, is haunted by Sylar, who claims to have lingered in Parkman's mind after Parkman "sucked" all his memories and replaced them with Nathan's. Though Parkman had resolved not to use his powers anymore, Sylar eventually tempts him into using them again. Following this, Sylar escalates his tormenting of Parkman, using his own telepathy against him to force Parkman to use his powers to save himself. Sylar then gives a defeated Parkman an ultimatum: either continue to live with Sylar in his mind or return Sylar's mind to his body.  Matt decides to keep Sylar in his mind, which begins to have consequences when Sylar's control over Matt advances, where Sylar has sex with Matt's wife Janice while Matt was asleep. Scared of what Sylar could do to his family (which Sylar himself alludes to), Matt tells Janice the truth, and Janice agrees to let Matt try to exorcise Sylar by leaving the house. During another conversation, Sylar appears pained whenever Parkman drinks alcohol, giving Parkman the idea to drink into a stupor. Sylar appears successfully removed as he fades from Matt's mind, and Matt remains conscious long enough to see his partner and Janice return. In actuality, Sylar simply tricked Parkman into knocking himself out. Now much more powerful, Sylar takes full control of Parkman's body, leaving Matt trapped in his own head. Despite his control over the body, Matt's personality constantly hampers Sylar from reaching his body, including letting Sylar be arrested and "withholding" Sylar's ability to use Matt's telepathy. In response, Sylar threatens to murder innocent civilians if Parkman doesn't cooperate. Faced with an ultimatum, Parkman attempts to commit suicide by cop.  The attempt appears to almost work, but "Nathan" and Peter arrive to revive Matt. Once revived, Matt is surprised to see Nathan and Peter, as is Sylar. Desperate to return to his original body, Sylar forcefully takes over Parkman's body, only needing to touch Nathan in order to jump back into his own body. While "Nathan's" confidence in who he is is shattered (Matt reveals that "Nathan" is Sylar), Sylar uses Parkman's telepathy to bring Nathan closer to him, with intuitive aptitude forcing Nathan to throw Peter telekinetically and proceed to get closer, to understand who he is. Although the event is temporarily stalled by an incoming guard, Nathan brushes hands with Matt, freeing him from Sylar's hold, and leaving Nathan fearful that Sylar will soon take over his body once more.

Q: What is Haunting Matt Parkman?
A: Matt Parkman, regretting his actions, is haunted by Sylar,

Q: What does Sylar do to Matt Parkman?
A: Sylar escalates his tormenting of Parkman, using his own telepathy against him to force Parkman to use his powers to save himself.

Q: How does Parkman react to the haunting?
A: Matt decides to keep Sylar in his mind, which begins to have consequences when Sylar's control over Matt advances,

Q: What kind of consequences begin when Sylar takes control of Matt?
A: Sylar has sex with Matt's wife Janice while Matt was asleep.
Does Matt find out about Sylar's actions with Matt's wife?
Scared of what Sylar could do to his family (which Sylar himself alludes to), Matt tells Janice the truth, and Janice agrees to let Matt try to exorcise Sylar
Scared of what Sylar could do to his family (which Sylar himself alludes to), Matt tells Janice the truth, and Janice agrees to let Matt try to exorcise Sylar