On Buffy the Vampire Slayer, Cordelia Chase (Charisma Carpenter) was a vapid high school mean girl who bullied the titular hero, making her life at school almost as rough as the time she spent staking vampires at night. However, Angel saw her progress into a legitimate hero, which is one of the stand-out parts of the spin-off.
During a reunion for the 20th anniversary of the vampire series, co-creator David Greenwalt explained to Entertainment Weekly that Carpenter asked if she'd be able to return to Buffy if Angel flopped. However, Whedon saw the the new series and Cordelia as an opportunity to tell a story he couldn't with Buffy, who started killing vampires pretty much as soon as her show started.
“You get to watch [Cordelia] go from somebody who is completely shallow and self-involved to somebody who is a hero,” Whedon said.
Spinning out of Buffy the Vampire Slayer, Angel ran on The WB for five seasons from 1999 to 2004, featuring several characters from the original supernatural series. Created by Joss Whedon and David Greenwalt, the series was the highest-rated program cancelled by the network in 2004, reportedly due to a request for an early renewal from Whedon. The characters and storylines have since been continued through licensed comic books published by Dark Horse Comics. BOOM! Studios is currently releasing stories of the Buffyverse in a rebooted reality.