Here’s what I think the Hogwarts houses of the different BTVS characters would be:

Gryffindor: Buffy Summers, Faith Lehane, Angel, Robin Wood, Connor, Charles Gunn, Allen Doyle, Kate Lockley

Ravenclaw: Willow Rosenberg, Rupert Giles, Kendra Young, Fred Burkle, Wesley Wyndam-Pryce, Jonathan Levinson, Andrew Wells

Hufflepuff: Xander Harris, Joyce Summers, Dawn Summers, Riley Finn, Lorne, Daniel Osbourne, Tara Maclay, Kennedy, Anya Jenkins

Slytherin: Cordelia Chase, Harmony Kendall, Spike, the Mayor, the Master, Drusilla, Caleb, Illyria, Warren Mears, Amy Madison