The Reaper - half Time Lord, half immortal human, the child of Captain Jack Harkness and an unknown Time Lady.
He only has enough regeneration energy to regenerate a single time, but due to being a fixed event in space time like his father, when he actualy dies he will come back to life as his original self. So in effect infinite regenerations.
He is trillions of years old, and has visited every single person in creation at least twice, once to know them and then again at the time of their death to save their conciousness into his virtual world 'The Library Core', via the use of his scythe.
At the end of this unviverse he has found a way to transfer all their conciousness's into the next universe, giving everybody a second chance at life. Only energy and not matter can be sent through to the next universe, so eventualy he will be left behind, spending all eternity on his own.
For The Reaper, when he is finaly and completely alone, Silence will fall!