The leader: Buffy
The scientist, who uses a lot computer screens: Willow
The one who knows the lore, who handles a lot of dusty old books: Giles
The comic sidekick: Xander.
The outlaw who’s allied with the good guys: Angel and later Spike
The muscle: Buffy, once again.
And of course, the Big Bad: A sympathetic villain whose story arc spans a full season or more. Examples: the Mayor in Buffy Season 3, Bulshar on Wynonna Earp.
A good Big Bad steals every scene he or she is in.
Roles of individual characters change over time, and are often combined in a single character. In Stargate SG-1, O’Neill was both the leader and the comic sidekick. Teal’c was both the outlaw and the muscle. In Buffy, Willow became a powerful witch. Doctor Who is leader, comic sidekick, outlaw, scientist and loremaster all in one.
Star Trek shows have similar but different roles. No Trek is successful without the Alienated Alien – for example, Spock, Data and Odo. Voyager was flailing until they brought on 7 of 9. (Credit goes to a friend on the Alienated Alien observation.)