That's a surprisingly tricky question because 'The Traitor' is a very common book title. Which one do you mean? There's a spy thriller by Andy Marino, a medieval fantasy by Seth Dickinson, a mystery by R.R. Haywood... It's a real needle in a haystack situation. My first thought always goes to Seth Dickinson's 'The Traitor' which is the first book in the 'Masquerade' series. The central character is Baru Cormorant, a brilliant, calculating woman from a colonized island who joins the Empire's administrative service to destroy it from within. The whole story is a devastating look at power, identity, and sacrifice through her eyes.
Baru is basically the only character who truly matters in that sense; everyone else orbits her grand, cold strategy. It's her internal conflict—between her mission, her suppressed desires, and the empire's corrosive logic—that drives everything. The supporting cast like Tain Hu, the Duke of Vultjag, becomes crucial as a foil and a potential emotional anchor, but the book is Baru's chessboard. You spend the whole time inside her meticulous, wounded mind.