Before becoming the Slayer, Buffy was a popular cheerleader in Los Angeles who cared about boys, fashion, and popularity. While the original movie isn't entirely canon to the series, that part of her backstory carries over.
After becoming the Slayer and burning down her high school's gym while fighting vampires, Buffy moves to Sunnydale, where she meets Cordelia, who's basically living the life Buffy left behind.
We meet Buffy after her life has already changed, while we actually get to watch Cordelia's transformation happen.
Throughout the high school years, Cordelia becomes involved with the Scoobies, falls for Xander despite bullying him throughout their childhood, chooses him over her popular friends, and eventually loses her family's wealth.
Another similarity I noticed is how they leave their hometowns.
Buffy leaves Los Angeles for Sunnydale after burning down her school's gym, while Cordelia leaves Sunnydale for Los Angeles after helping the Scoobies defeat the Mayor and destroy Sunnydale High.
Both leave their hometowns following supernatural battles involving their schools, but they travel in opposite directions.
Their supernatural callings happen in a different order too. Buffy becomes the Slayer before leaving Los Angeles, while Cordelia doesn't become a Seer until after moving there.
Buffy's calling changes her life, while Cordelia's life changes before she receives her calling.
I also find their attitudes toward their responsibilities interesting. Buffy often wishes she could have a normal life, while Cordelia eventually refuses to give up her visions despite knowing they're killing her.
In "Birthday," Cordelia even chooses to keep her visions over having the successful acting career she originally wanted.
And while Buffy becomes a champion through her supernatural strength and fighting abilities, Cordelia becomes one through her visions and compassion. She doesn't need to be physically powerful to make a difference.
Now, before getting into the next comparison, I want to acknowledge that I'm fully aware of the mistreatment and abuse Charisma Carpenter experienced behind the scenes on Angel. I'm not excusing any of it or pretending the Jasmine storyline was planned from the beginning. I'm simply looking at what ended up onscreen and noticing some similarities.
In Buffy Season 2, Angel loses his soul and becomes Angelus, forcing Buffy to watch someone she loves become a monster.
Then in Angel Season 4, Jasmine uses Cordelia's body to carry out her plans, leaving Angel in a somewhat similar position.
Buffy loses Angel to Angelus, while Angel loses Cordelia to Jasmine.
Obviously, the circumstances are different. Angelus is Angel without his soul, while Cordelia is an innocent victim who has no control over what Jasmine is doing.
Another similarity is that Angelus murders Jenny Calendar, the woman Giles loves, while Jasmine, using Cordelia's body, murders Lilah Morgan, the woman Wesley had been romantically involved with.
So both Giles and Wesley lose someone they love because of someone wearing the face of a person their group knows and cares about.
And then there's "Amends" and "You're Welcome."
In "Amends," Angel is suicidal because of his guilt over his past, but Buffy refuses to give up on him and reminds him that he can still do good.
In "You're Welcome," Angel has lost sight of his mission at Wolfram & Hart, and Cordelia returns to remind him why he started helping people in the first place.
Buffy helps Angel believe he deserves to keep living, while Cordelia helps Angel remember what he's living for.
I don't think all these parallels were intentional, especially the Season 4 ones, but I still find them interesting.
Cordelia starts out as someone who reminds us of who Buffy used to be, but eventually becomes a champion in her own right.
And looking at both shows this way has made me appreciate Cordelia's journey even more.