A ancient chinese vampire.
He was rumored to have challenged his prey to test his skills. In the 20th century he showed up in Sunnydale to challenge the local kung fu trainer into mortal combat. But Buffy (the current slayer) interfiered as San-Sui was about to finish him off. San-Sui is about to kill Buffy when Xander stakes him from behind "killing" him.
About 3 years later he was supposed to fight Buffy in a demon fighting arena arranged by Adam.