Even before he knew what they were, Harth had the dreams.
The twin brother of Melaka Fray, he inherited the memories and dreams of a Slayer - while his sister gained the strength and healing ability. He knew that Melaka was the Slayer long before she did, and when she was unable to stop a vampire draining his blood, also knew how to survive. He deliberately bit the vampire killing him, and became one himself.
It turns out that there was a prophecy about him. (There's always a prophecy.) Harth Fray is 'the One who Will Lead' - that is, lead the vampires in a crusade to bring about Hell on earth.
That doesn't mean he doesn't like to check up on family now and again, however... He did miss Mel in the four years from his death to his reappearance. His brotherly love for her when he was alive seems to have twisted into a kind of obsession with hurting her to prove his love.
Harth appears to be about fourteen years old, as this was his age when he died - in reality he is (naturally) the same age as Mel. He has dark hair and blue eyes, and looks unassuming but pleasant. When in vamp-mode, his face sharpens unnaturally, fangs appear, and his eyes burn golden. In personality, he is quietly confident, more than a little creepy, and enjoys mind games more than physical torture - although a healthy bit of pain generally helps those along.
Harth Fray is a character from Joss Whedon's Fray. Not mine. No profit. Just fun and games in milliways_bar.