Shell, Elphaba's little brother, barely appears in the novel, which is really significant. He only shows up in one scene at the end, where he's checking on a sleeping Frex (5.18.5). The rest of the time, we only hear about him secondhand from Elphaba, Nessa, and Frex. This really is a story about "two sisters," as Yackle says. Shell is almost an afterthought.
But he's also like a successful version of Elphaba – the underground hero who is forgotten by history while his sisters go down in infamy. Shell is always running around like some cool super spy, fighting the good fight against the Wizard and being heroic and exciting. Compared to the rest of his family, Shell might actually be one of the best adjusted Thropps. He doesn't end up terrorizing people or performing questionable acts of magic, at least, and that's saying something in this novel.