The group practiced small magics for pleasure and gain, until Ethan and Ripper discovered something bigger: the demon known as Eyghon, or the Sleepwalker. Tattooing themselves with the Mark of Eyghon, they would take turns falling asleep, and the rest of the group would summon the demon into the sleeper. According to Giles, it was an extraordinary high, a euphoric feeling of power, but was also incredibly risky. When Eyghon took control of Randall, one of their group, the others tried to exorcise the demon, resulting in Randall's death. Ripper was changed by the event, leaving London and returning to the Watchers' Council. Ethan, on the other hand, went the opposite direction, delving deeper into the black arts. When Ethan asked, years later, when things had changed, when they stopped being mates, Giles replied, "When you started to worship chaos."
Opening a small costume shop in Sunnydale's business district around October 1997, Ethan curses his costumes in the name of the Roman god Janus, so that the wearer becomes whatever they are dressed as. For example, once the spell is cast, Xander Harris becomes a soldier, Buffy Summers becomes an 18th century noblewoman, and Willow Rosenberg becomes an actual ghost. Upon discovering that Ethan is responsible, Giles describes the spell as "sick, brutal, and it harms the innocent", and proceeds to beat Ethan until he reveals how to break the spell.
Later, in the episode "The Dark Age", Ethan comes looking for Giles in the Sunnydale High School library. He has been having dreams of Eyghon, and the fate of the others who wear "the Mark." Ethan knows that somehow the demon has returned again and is roaming free upon the world. The Mark of Eyghon on the original group members acts as a beacon for the demon to locate them, kill them, and possess each corpse in turn to hunt his next victim with. Ethan plans to 'hide behind the Slayer', exploiting her desire to protect Giles and thus protect Ethan in the process. Knocking Buffy unconscious, he waits for her to wake up before tattooing the Mark of Eyghon on the back of her neck and removing his own Mark with acid. The substitution works, and the demon loses interest in him, instead going after Buffy. But Giles arrives, and then her friends, who stop Eyghon from killing both of them.
In the Season Three episode "Band Candy", the vampire Mr. Trick, in the employ of Mayor Richard Wilkins, later calls on Ethan's services when he needs Sunnydale's adult population distracted during a tribute ceremony for the demon Lurconis. Ethan curses candy bars that are distributed to adults through Sunnydale High School students. The curse on the chocolate makes adults act as if they are teenagers, leaving many areas of the city unprotected, including the hospital where the tribute of small babies needs to be taken from. Buffy and Giles uncover the plot, and stop Mr. Trick and Ethan (who claims ignorance of the nature of the tribute), but they both escape.
In the fourth season, Ethan returns to Sunnydale in the episode "A New Man." Discovered lurking in a crypt by Giles, he talks him out of a promised beating, and instead, they go for a drink together. Ethan warns Giles that the Initiative is throwing the worlds out of balance, which goes "way beyond chaos", into "one hell of a fight." Giles doesn't seem to take the warning seriously, instead being somewhat envious of the Initiative. After spending the night reminiscing and drinking with Ethan, Giles wakes the next morning in the form of a Fyarl demon. After the spell is reversed, Ethan is arrested by the Initiative, who apparently place him in military custody pending determination of his status, before sending him to a rehabilitation facility in Nevada.
While infiltrating a military installation two miles (3 km) south of Sunnydale, Buffy attempts to break Ethan out of his cell, but instead finds him dead, having been shot and apparently killed by General Voll in "The Long Way Home, Part Four." The fact that he aided Buffy before his death arguably makes him an example of the series' strong theme of redemption.
However, his fighting skills are mediocre, as Giles repeatedly beats him up with little-to-no effort; whenever Giles does this, Ethan doesn't even try to fight back.
He also appeared in the final book of the Buffy Gatekeeper Trilogy. He came to Sunnydale and summoned the Lord of Hell Belphegor via a magic mirror but when Belphegor rejected him he helped Buffy and the gang. He was there for the final battle although he and Giles were both quickly knocked out but his subconscious reached out to Buffy and showed her what he had learned on how to defeat and destroy Belphegor. She used this information and Belphegor was destroyed and the world saved. He left soon afterwards.