Vincent Valentine is probably the most favourite optional character of all time. He was originally a secret character in Final Fantasy VII, along with Yuffie Kisaragi, a ninja who also appears in the game that Vincent stars in, Dirge of Cerberus.
Vincent Valentine first appeared in the basement of Shinra Headquarters in a coffin on the game Final Fantasy VII for the PlayStation.
Vincent wears a torn red cloak, of which the top covers half of his face and the whole thing is held on by a cetain number of buckles. Underneath his cloak he wears pure black and a sort of bronze coloured pair of boots. On his left hand is a gauntlet (which ends with talons), of which the reason for is still being pondered over by many fans, but from images of Advent Children and Dirge of Cerberus you can see a black glove underneath it, so it is believed to be worn by free will. His weapon of choice is a three-barrelled(sp?) gun called Cerberus, which has a Cerberus emblem hanging from a chain on the handle which gives it it's name.
His name comes from a mixture of Vincent Price and Saint Valentine.
Vincent is an outsider, who prefers to be by himself. He is tall and dark, and most of his past is a mystery, though the events of Dirge of Cerberus give information on it, as he is the main character of it. I think he is the only optional character I have ever heard of who has made it to being a main character.