Lord Ruthven first appeared in the 1819 short story "The Vampyre" (which was thought up on the very same night that Mary Shelly created her Frankenstein story) by John William Polidori.
He's a man of mystery who has managed to penetrate the High Society of London. It is in these circles that he meets Aubrey, a wealthy young gentleman whom he soon befriends. The two travel throughout Southern Europe, where a misshap involving bandits seems to take the life of Ruthven, but not before the Lord makes his young friend swear an oath not to reveal the circumanstances of his death to anyone. To add more mystery to things, Ruthven's body seems to disappear overnight. As he travels back home, Aubrey begins to piece together various seemingly unrelated facts that seem to indicate that Ruthven was not who he seemed to be. He begins to suspect that Ruthven may in fact be... a VAMPIRE.
This is only compounded when, upon returning home he runs into Ruthven, not only seemingly back from the dead (under a different name) but intending to marry his sister. The elusive Lord manages to thwart Aubrey's every attempt to warn his sister (reminding the younger man of the oaths he took) of the peril she was in. Despite his best efforts, Aubrey's sister marries Ruthven, and tragedy soon strikes afterwards.
The character of Lord Ruthven became popular for quite some time after the initial story (appearing in three subsequent sequels in the years following). While he soon became overshadowed by the likes of Count Dracula, he's still managed to make appearances in various stories since then. He's appeared in a few Marvel comics, and also has a DC Comics counterpart that shares his name.