Seth Stringer (also known as Orc Of Oppression and Revolution) makes his first appearance in chapter 9 where he is being bullied by other high schoolers. As an avid fan of William Blake he is often seen with his book or quoting pieces by William Blake.
At his first appearance, Seth was believed to be acting under the orders of Professor Eames who lead a cult, but instead Seth revealed himself to be the one who was controlling Eames all along. Seth is a contractor of Orc who uses a weapon called Sluagh Ghairm which takes the form of a horn that allows him to control the expansion and contraction of the air including spatial interference.
He is a teenager who made a agreement with the devil to obtain his power and to fight with Arago. His ultimate goal and his reason for needing the Brionac (despite the fact that he can't hold its power) is unclear, however he did said "in order to eliminate the people and the system that have built this world of deception and lies" during his first fight with The White Horseman.