First off, I'm gonna assume we're talking about 'Red Rising'? Because 'boot camp book' could fit a few, but that's the one that immediately clicks for me as a boot camp narrative from start to finish.
Darrow is the obvious core, the Red who becomes a Gold. The story lives and dies with his rage and his transformation. Then you've got Mustang, who's just... she's the brains and the moral compass, but never in a boring way. Cassius is the golden boy who becomes a rival, and Sevro is the wild card, the ugly little genius who steals every scene he's in.
Honestly, the first book's cast is a bit more contained than the later ones. You spend most of your time with Darrow's squad in the Institute, watching those alliances and hatreds form. The Jackal is the villain you love to hate from the start, a quiet, calculating threat. Roque is the poet, the sensitive one. It's this microcosm of society, and Pierce Brown makes you care about almost all of them, even the ones you're supposed to dislike.