Andrew Wells is the younger brother of Tucker Wells, who summoned hellhounds to attack the Prom in the Season Three episode "The Prom". Andrew himself has skills in demon-summoning and animal control, although these were never used after Season Six;
Andrew is only introduced (long after the destruction of the school) in Season Six, as a member of the Trio, Buffy's self-styled "arch-nemeses". Warren, the leader of the gang, is almost entirely driven by greed and lust for power; whereas Jonathan apparently joined the gang for excitement, being shocked by Warren's ruthlessness and eventually helping to save Buffy's life. Andrew occupies a sort of middle ground between these two, initially, as before, simply seeking power and amusement, but becoming more evil as the show progresses, for example, cheering "kill her!" as Warren fights Buffy.
Returning to Sunnydale in Season Seven, Andrew murders Jonathan in an attempt to open the seal, but is soon discovered and held hostage by the Scooby Gang. At first, the Scoobies are hostile towards Andrew, partially because he murdered Jonathan, partially because of his alliance with Warren and partially because of his annoying habits. Andrew provides much of the comic relief in Season Seven along with the advancement of major plot points. In his own episode "Storyteller", Andrew decides to document the Scoobies' adventures for future generations using a video camera. Andrew has a tendency to exaggerate, or tell his own version of the "truth," routinely relating overly-dramatic personal narratives. Although Xander, Anya, Spike, and Dawn are happy to let him interview them, Buffy is angered by his attempt to turn everything into a story and forces him to finally face up to the seriousness of his actions. After this, he becomes somewhat remorseful and repentant, joining Buffy as a comic sidekick in the fight against the First, surviving the apocalyptic battle at the end of Season Seven.
Andrew returns in two episodes of Angel's fifth season, and he appears to have matured slightly. Now under the mentorship of Rupert Giles, who is training him as a Watcher, Andrew comes to Los Angeles to help contain psychotic Slayer Dana.