Seth is Abbigale's abusive ex boyfriend who makes his first appearance in chapter 1, when James sees him kissing Abbi. In chapter 2, he is finally introduced to the reader when James approaches him for yelling at Abbi. She breaks-up with Seth with James by her side.
Seth seemingly disappears until Chapter 7. Seth goes postal and shoots up Lakewood High, killing 52 people. James assumes Seth snapped after is break up with Abby. Seth is finally taken down by Jason who ultimately beats him into a coma. It was never stated if Seth woke up from his coma, although it is briefly mentioned that he is behind bars.
Later in the book, Abbi reveals that Seth was physically abusive in her confession letter she wrote to James. In the letter, she tells James that Seth figured out that she became pregnant from a previous gang rape. After this realization, he decides to beat the unborn child out of her by repeatedly punching her in the stomach.
Although Seth's physical appearance is never described in the story, the outfit he wore to shoot up Lackwood High was. He wore a long white coat with a white outfit underneath. James believes Seth planned to wear this outfit so the blood of his victims will cover his body.
Greg and Seth
The original Stones To Abbigale cover was the front of Seth's outfit with blood stains on it. For the second book cover, Greg re-created Seth's outfit and modeled Seth holding a gun. After the final cover change, Greg kept Seth on the back cover.
In a video titled, "Why Are People Such Jerks Online?", Greg read a negative Amazon review for STA that said the characters are not interesting. Greg shows off Seth's outfit and uses Seth as an example of an interesting character. ($ource)
Greg continued to wear Seth's outfit in several Speaks and UhOhBro videos after his book was released.