Seth has been around Herrick for a long time. He was with Herrick in 1917 when Mitchell was recruited. In Flotsam and Jetsam Mitchell mentions towards Seth that "the coolest kid in school is suddenly his mate" (meaning Herrick). This means that Herrick and Seth weren't mates before, they just spent some time together. Seth is just a replacement for Mitchell when Mitchell is not around. Herrick prefers Mitchell. He even mentiones that Seth is an idiot. Seth knows that Herrick likes Mitchell a lot more and is jealous because of it. He tries to please Herrick, but never gets the same acceptance as Mitchell.