Tub is the protagonist. He is the first character introduced, and the story is told from his perspective. He is not intended to be a hero character, but simply the one whose perceptions color the story; he accepts Kenny's abuse until the possibility of violence comes up, and then he defends himself from what he considered a personal threat. Tub is not very sympathetic, especially when he reveals that he deliberately overeats:
"...I make a big deal about only having an orange for breakfast and then scarf all the way to work. Oreos, Mars Bars, Twinkies. Sugar Babies. Snickers." Tub glanced at Frank and looked quickly away.
(Wolff, "Hunters in the Snow," classicshorts.com)
In the end, Tub bonds with Frank because of their shared secrets, and together they make an unconscious decision to let Kenny die for his abuse. This shows the callous nature of Tub's inner personality; he accepts Kenny's abuse as validation, but when he receives validation from Frank, he stops caring if Kenny lives or dies.