Answer:
the main characters (in every, or almost every, episode):
Buffy, Willow Rosenberg, Xander Harris, Rupert Giles.
characters that come in later in the series or were only in the beginning seasons but are still considered "main":
Joyce Summers, Angel, Jenny Calendar, Cordelia Chase, Faith, Anya, Tara MaClay, Oz, Wesley Wyndam-Pryce, Dawn, Spike, Riley Finn, Andrew Wells, Warren Meers.
on and off characters:
Jonathan Levinson, Harmony Kendall, Drusilla, Darla (only season one), Ben (only season five), Amy Madison, Ethan Rayne, Kendra the vampire slayer (only season two).