Seth Ryan is a minor character in American History X who becomes a brief antagonist during Derek's confrontation with D.O.C. - a Neo-Nazi gang that Derek had created prior to going to prison but later opposed after it was taken over by Cameron Alexander.
Seth was a large individual but friendly towards Danny, Derek's younger brother, due to the pair sharing racist Neo-Nazi beliefs - this would climax in Danny joining the D.O.C., much to Derek's disgust after he returned from prison a new man.
Seth would attend the same Neo-Nazi rally as Danny and Derek when Derek had his infamous confrontation with Cameron Alexander, sensing something was wrong when Derek left Alexander's office Seth confronted Derek verbally before rushing into the office.
Upon seeing that Derek had beaten Alexander Seth flew into a rage and pulled out a gun, threatening to kill Derek as the other gang members watched - however Danny's pleas stalled Seth just long enough for Derek to disarm him and steal the gun, the two brothers then fled the D.O.C.
It is later revealed that Seth had been hospitalized alongside Alexander. However, in an early draft of the film, he was shot and killed by an African American along with Cameron, who was killed from a curb stomp by another African American, believing that Cameron was Derek.