From Heroes Wiki
Dennis was a Level 5 escapee whom Sylar hunted down.
Sylar tracks the three level 5 escapees, one of them Dennis, to Rachel's apartment and goes in expecting a fight.
While in Rachel's apartment along with fellow Level 5 escapees Tina and Michael, Dennis greets Sylar, and asks if Sylar had come to join them. Sylar says no and kills Dennis, only leaving his burning skeleton.
Later, a number of other Company agents arrive to remove Dennis's body.
Please refer to Theory:Dennis for fan-created theories and other speculation.