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.
Memory loss
Six weeks later, this Nathan starts to rediscover the other powers he had gained in his old life. Later, following Nathan's confession of being involved in the accidental death of his girlfriend Kelly Houston, her mother, Millie Houston, has him shot and buried in a shallow grave. When healed, Sylar emerges from the ground in his own body, suffering from amnesia. Picked up by the police, he is recognized and accidentally uses his powers to escape. He then encounters the "Sullivan Bros. Carnival" and Samuel, who take him in as one of their own. The carnival is hidden from Sylar's pursuers, and Samuel assures him he is safe in his new "home".  Once Sylar has rested, Samuel proceeds to show him around the circus, showing that everyone at the carnival has an ability. Sylar also meets Lydia, who he immediately hits it off with. Edgar, however, jealously attempts to outshow Sylar with his power, but is easily beaten. Samuel also notices that Sylar's memories are not his (in fact, Nathan Petrelli's). In an effort to awaken Sylar's true personality, Sylar is taken by Damien to the hall of mirrors, where he is forced to look at his past murders, starting with his mother. Repulsed, Sylar is unable to believe himself a monster. Samuel uses this to try and convince Sylar that he is a protector of the circus, and must remove threats such as the officer who attempted to capture him the night before. Sylar is told the officer is at the house of mirrors, and goes there to tell the man to leave. However, the officer shoots at Sylar, forcing an uncontrolled electric blast. Here, Sylar's killer instinct begins to take hold, but Sylar stops himself just in time for Edgar to cut the man into pieces. Later, Samuel formally makes Sylar a part of the circus family, baptizing him and letting him mingle with the others. Sylar and Lydia leave arm in arm for the night, much to Edgar's dismay.  In "Shadowboxing", the persona of Nathan returns, causing him to shapeshift back into Nathan's form while sleeping. Panicking upon finding himself at the carnival, he flies away, and is later shown arriving at Peter's door, asking for help. Arriving for work at his office, Peter and Nathan discover that no one has been looking for Nathan, as he had been on 'vacation' according to Angela Petrelli. The brothers are further surprised when the Haitian appears, and tells Peter to go see a storage facility for the truth of the situation. Here, Nathan and Peter discover the real Nathan's preserved body, which 'Nathan' touches and sees Sylar and Matt Parkman. They head to Texas to see Matt, who is at the hospital, in a coma.

Q: How did he lose his memory?
A: shot and buried in a shallow grave.

Q: Did he lose all his memory or just some?
A: Sylar emerges from the ground in his own body, suffering from amnesia.

Q: Does he ever regain his memory?
A: unknown
Are there any consequences to his memory loss?
he is forced to look at his past murders, starting with his mother.
he is forced to look at his past murders, starting with his mother.