1) Tara Maclay
Tara comes to the show late and leaves so much earlier than she should have. She is the most moral and sweet person in the cast. At first, her timidity and uncertainty act as a barrier between her and the Scooby gang. Her abusive family fed her lies like they did all the women in her family and the effects showed in her behavior. She expected people to be mean to her because that's how she was treated for most of her life. When she grows closer to the group and finally realizes she isn't a demon, the others come to her for advice. Tara has a quiet strength that isn't physical like Buffy and isn't in magic like Willow, but her resolve and moral compass are formidable. She doesn't tell Glory anything despite her god powers. The dark side never tempts her because she doesn't want or need it. When Willow becomes abusive, Tara gives her opportunities to change and leaves when Willow continues to lie and abuse magic. She doesn't allow herself to be abused anymore and leaves no matter how much it pains her to be separated from Willow and their friends. Tara shows that strength doesn't have to be loud or obvious.
2) Cordelia Chase
Cordelia starts the show as a queen bee mean girl who is cruel to Buffy, Willow, and Xander. She serves as a foil for Buffy, showing what Buffy could have been had she not been the slayer. Over time, it shows that Cordelia uses cruelty as a cover to protect herself. Surrounded by her Cordette toadies, she feels alone and knows that people only spend time with her to be popular by proxy instead of actually caring about her. Ironically, the Scooby gang, who spent so much time being tormented by her, were some of the first to accept her for who she is and not what her status could bring them. Throughout the series, she never loses her edge of brutal honesty, shown explicitly in the Earshot episode when she's the only one to say exactly what she thinks. Her character blossoms more on the Angel series (except for season 4), but the foundation of a subversive woman who asserts herself, feels confident, and gets what she wants.
3) Faith Lehane
Faith is the third slayer seen on the show, called after Kendra is killed. Her life has not been easy as she's been living alone, supporting herself with a healthy mistrust of others. The others see her stories as something to be envied but she doesn't share with them the harsh realities she experienced. Underneath her bravado, she's in pain and lonely. Her relationship with Mayor Wilkins is the first loving and supportive ones maybe ever, so it's no surprise that she switches sides. After being defeated and left comatose, she awakens to magically steal Buffy's body and finds a moral center when parodying and parrotting Buffy's goodness. She allows herself to be taken to jail to serve her sentence even though she could easily escape. Faith's road to redemption wasn't as easy as Willow's because her support system was gone. The Scoobies would never accept her as she was because she didn't have the same connection. Faith is a strong, brash person who made mistakes on the way to finding friends and family. She returns in the final season not as an enemy, but as a friend who challenges Buffy's decisions. I love her character arc. She's the only character to go from being good to being so entrenched in evil and back.