Warren Mears was played by Adam Busch.
Warren began on the show as a seemingly one-off character, involved in the main storylines of two Season 5 episodes. The character was reintroduced in Season 6 as part of the Nerd Trio, where he progressed from a comic annoyance to a misogynistic killer driven by an urge to dominate and his growing mental instability, and one of the show's major villains, though not a Big Bad.
Warren in Season 5
Warren's first appearance was in "I Was Made to Love You," in which he was a former Sunnydale student who built a robot girlfriend for sex and company. In the episode, the robot April searches for Warren, who has abandoned her for a human girlfriend, Katrina. April's batteries eventually run down, but not before she attacks Katrina. As Warren is about to leave town to try and reconcile with Katrina, Spike approaches him with a request. In "Intervention," Warren fulfills Spike's request - the Buffybot. Warren then dissapears until Season 6.
Warren in the First Half of Season 6
Warren resurfaces in the Season 6 episode "Flooded" as part of the Nerd Trio, along with Jonathan and Andrew. During the first half of the season, the Trio appears as a comic nuisance to Buffy, who gets in the way of their plans to rob banks and so forth.
From the beginning, it is clear that Warren (unlike his more passive portrayal in Season 5) is the leader of the Trio, dominant over Jonathan and Andrew, as well as more willing than the other two nerds to take real risks and possibly do real harm to people.
Warren in the Second Half of Season 6
Starting with the episode "Dead Things," Warren shows his true colors as a dangerous, misogynistic and possibly psychopathic individual. When Katrina reappears, Warren uses an enslavement device to make her obey his will. When she resists the device, Warren attacks her and accidentally kills her. Unmoved, he creates a scheme whereby he nearly has Buffy blame herself for Katrina's death. Buffy figures it out, but Warren and the Trio evade her for the time being.
After Katrina's death, Jonathan becomes increasingly unhappy with Warren's lack of conscience. Warren plots with Andrew, who is enamored with him, to ditch Jonathan in the near future.
In "Seeing Red," Warren makes his bid for major villain status when he steals a pair of magical orbs which give him strength and invulnerability. He, Jonathan and Andrew attempt to rob an armored car, but Buffy (aided by Jonathan who surreptiously tells her to destroy the orbs) stops him. However, Warren escapes with a jetpack. Andrew has one too, but he blasts off into an overhang, and he and Jonathan (who received no jetpack as part of Warren and Andrew's plan to abandon him) are jailed. In prison, Andrew laments over Warren leaving him behind.
The next day, Warren, upset at his plans being foiled, appears in Buffy's back yard with a gun. He shoots, wounding Buffy and accidentally killing Tara. This action creates the season's true Big Bad, as Willow is pushed by grief into dark magics and becomes Dark Willow. She hunts Warren down and flays him in retribution for Tara's death, ending Warren's life and his villainy.
Warren in Season 7
The First Evil appears as Warren a few times in Season 7 - including as Season 6's representative in the parade of villains at the end of "Lessons-" using his appearance mainly as a means of bending Andrew to its will. Using this method, the First Evil gets Andrew to murder Jonathan (in Conversations With Dead People), in order to open the seal in the basement of the new Sunnydale High.
Warren's final major appearance occurs in when a spell (cast by Amy) causes Willow, feeling guilty over moving on after Tara's death, to manifest her feelings by making her outwardly assume the appearance of her former victim.
Warren appears in the following episodes:
Lessons (as a manifestation of the First Evil)
Conversations With Dead People (the First Evil)
Never Leave Me (the First Evil)
The Killer in Me (Willow appears as Warren)