Andrew
Although presumably attending Sunnydale High along with Buffy (most likely a grade or two below her), Andrew was only introduced (long after the destruction of the High School) in season six, as a member of the Trio, Buffy's self-styled "arch nemesises"Warren Mears, the leader of the gang, was almost entirely dominated by a lust for power and greed; whereas Jonathan Levinson always had doubts and eventually helped save Buffy's life. Andrew occupied a sort of middle ground between these two, becoming more evil as the show progressed, for example, cheering "kill her!" as Warren fought Buffy, and actually killing Jonathan in season seven. After this Andrew became somewhat remorseful and repentant, culminating in the episode "Storyteller". In this episode Buffy forces him to face up to the seriousness of his actions. Afterwards Andrew gradually began to try and help Buffy in the fight against the First Evil. Thus, the character of Andrew is an example of the show's strong emphasis on redemption. In two episodes of Angel's fifth season Andrew returns, and the character appears to have matured slightly. He is now working for the Watchers Council under the guidance of Rupert Giles.
A running joke in the show is that for a while nobody knew who Andrew was, and he was often referred to as "the other guy", "Tucker's brother", and so on.
credit: wikipedia
Episode List