Warren was a student at UC Sunnydale at the same time as Buffy with incredible talents at designing and building lifelike humanoid androids. Warren constructed "the perfect girlfriend", and android named April who is programmed to unconditionally love Warren. Warren later meets Katrina Silber, and the two begin to date. Rather than attempting to explain what is happening to April, reprogram her, or decommission her, Warren decides to simply leave her locked in his dorm room until her internal battery runs down. Rather than waiting, April became bored and decided to seek out Warren for herself. She went across the campus asking people if they knew Warren, causing physical harm to anyone that got in her way. April catches up with Warren at a local club, but he escapes with Katrina before she could locate him.
The rampaging April caught the attention of Buffy and the Scooby Gang. Since there was only one Warren in the school at the time, tracking him down did not prove difficult. Buffy came to Warren's house while he was trying to skip town, and forced him to help her stop April. Buffy took Warren to help her track down April.
One of the earliest signs of Warren's bad side came out when he and Buffy were tracking April. Warren told Buffy that April had to answer when he called out to her because of a feedback-loop that he installed in her. If she didn't respond quickly this feedback would flair up, causing her pain. Before Buffy and Warren found April she ran into Katrina. When April learned that Katrina was also dating Warren she became confused, and picked Katrina up by the neck. Buffy and Warren got to April just in time, and were able to get Katrina to safety. She was not badly injured, simply knocked unconscious from a lack of oxygen.
Buffy fought off the super-strong April, and won. When Katrina learned of what Warren had done she became disgusted with Warren and broke up with him immediately. Warren was later approached by Spike, who insisted that Warren build him a love android that was an exact replica of Buffy. Fearing for his safety Warren constructed the android, which would later go on to take her place during Buffy's death.
After meeting up with a Jonathan Levinson and Andrew Wells the three formed a group named the Trio. Jonathan had some experience with Magic, and Andrew experience summoning demons (he had trained a pack of flying monkey demons to attack the high school play earlier). They figured that they could use their combined skills and abilities to steal and extort wealth and power. Their first mission as a group involved a demon that they hired to help them rob a bank where Buffy happened to be applying for a loan at. While fighting with the demon Buffy wasn't able to stop the robbery, but she angered the demon who sought to take it out on the three. After the bank incident Warren and the others decided that Buffy was their arch-nemesis, and that part of their time would be spent plotting her downfall.
Buffy barely noticed the Trio at first, their antics being better classified as an irritation more than anything else. They set out on a mission to try to determine whose skills would be best suited to lead the group by seeing who could interfere with Buffy's life the most. Later, after an invisibility gun Warren developed went haywire, they turned Buffy into an unstable, invisible state. Buffy learned of what they were doing, and was able to stop them and turn herself visible before any permanent damage was done.
Things began to really go downhill for both Warren and the Trio when Jonathan developed a spell that would put any woman they chose under their control. Warren, who had began to push the others around earlier, insisted that he would be the first to try out the spell. He located his ex-girlfriend Katrina at a bar, and tried to pick her up. She responded saying that she wanted nothing to do with him ever again. In the face of this rejection, Warren used the spell on her and took her back to the basement where the Trio resided. While trying to rape Katrina she came out of his control. The spell that Jonathan had created was not powerful enough and she attempted to escape. Warren stopped her by smashing a glass bottle on her head, killing her. This made both Jonathan and Andrew want to leave, but Warren insisted that they were in too deep at this point, and forced them to help him dispose of her body.
The three decided to pin the murder on Buffy. Andrew summoned a group of demons whose mere presence alters the perception of time to anyone close in their proximity. While they were fighting Buffy this time anomaly occurred. When Buffy found the dead body of Katrina she believed that she had killed Katrina during the fight. Buffy was distraught over the incident and wanted to turn herself in to the authorities. Warren and the others were relieved that they had gotten away with murder until they realized that they had not fooled Buffy, and they were forced to go underground.
Warren knew that at this point Jonathan was less willing to go along with the plans that Warren made, and would likely tell on them. Andrew was still easily manipulated by Warren, believing Warren to be his close friend. Warren came up with a scheme that would take care of both Jonathan and Buffy. The three went to a demon-guarded cave, and inside stole two orbs which would make the bearer invincible and provide them super strength. Again, Warren decided that he would be the first to use them. Warren took Jonathan and Andrew around town where he picked fights with the largest people he could find. During an armored car heist, Buffy caught up with the Trio. Warren began to fight with Buffy, insane that a woman had caused them so many problems. Buffy shattered the orbs leaving Warren powerless. Before she could take him to the authorities Warren revealed a jet-pack he had under his jacket and flew off. Andrew revealed a similar jet-pack which revealed to Jonathan that the two had set him up. Andrew attempted to fly off with Warren as a big, dramatic gesture, but hit an awning, rendering himself unconscious. Buffy took both Jonathan and Andrew into custody.
Enraged that everything he had was falling apart, and blaming Buffy for the mess, Warren purchased a handgun and went to Buffy's home. He shot Buffy through the chest, and a stray bullet went through a window in the house killing Willow's girlfriend Tara. Warren fled thinking that he killed Buffy. Willow was distraught over Tara's death and attempted to bring Tara back to life. Since Tara's death wasn't due to mystical forces, the gods would not permit returning Tara to life. Willow absorbed as much dark magic as she could and went after the Trio.
Warren learned that he had not actually killed Buffy, and when he learned that Willow was after him he sought protection from everyone in the underworld that he could think of. No one would take him in. He sent an android that resembled himself on a bus out of town as a decoy, but this served only to slow Willow down briefly. Willow caught up with Warren in the woods and began to torture him. When Buffy and Xander arrived to stop her, Willow ripped Warren's skin from his body. Warren's body disappeared, and everyone presumed him to be dead.
Warren remained alive, summoned away from Willow seconds after his skin was ripped off by Amy Madison who used magic to restore him to life. Warren must have been dead at least momentarily because later a force named "The First" impersonated him, and The First could only take the form of someone who had died. Warren was alive without his skin, protected by Amy's magic. The two remained in Sunnydale until its destruction by Buffy and the other Slayer. When the town imploded Warren and Amy were trapped underneath the rubble.
Warren and Amy were both discovered by the government during an exploration of the ruins. They took both Warren and Amy in who agreed to help capture Buffy. Amy located Buffy in Europe, and teleported herself and Willow to the Army installation just out of Sunnydale where they were being kept. They knew that Buffy would be on her way shortly behind, and were permitted to keep Willow to do with what they pleased.
Warren was pleased to be able to torture Willow for removing his skin, but when Buffy arrived the Army was unable to stop her. She located where Warren and Amy had Willow, and rescued her. Amy teleported herself and Buffy away before anyone could stop them.