Vicious is a high-ranking member of the Red Dragon crime syndicate, and makes his first appearance in a shiver-inducing scene at the beginning of the fifth episode, Ballad of Fallen Angels. The episode not only reveals Spike's past ties to the Red Dragons, but also to Vicious. Although now mortal enemies, they were once close friends.
Little is actually known about Vicious, but hints of a love triangle and a betrayal offer reasons for the antipathy between Spike and Vicious. The mysterious pasts of the main characters, and a sense of sadness about friendships gone wrong, is part of what makes the series so fascinating. There's just enough clues about Vicious to indicate he wasn't someone who trusted easily, so the betrayal of Spike and Julia, two people Vicious had let close, played a large part in his downward spiral into violence and vengeance. I think this is best shown in Jupiter Jazz II when Gren tells Vicious he trusted and believed in him, and Vicious replies: "There was nothing there to believe ... there's nothing in this world to believe in. Nor is there a reason to believe."
I've written a little more about Vicious at my Bebop Musings site, especially regarding his relationship with Julia and whether or not he was trying to kill her or protect her at the end.
About Cowboy Bebop
Shinichiro Watanabe, Keiko Nobumoto, Yoko Kanno, and Toshihiro Kawamoto are part of the collaborative team behind Cowboy Bebop. The 26-episode series first aired in 1998, and was followed up in 2001 by a movie, Knocking On Heaven's Door (renamed Cowboy Bebop: The Movie in the US.)