The key characters really depend on how far into 'The Search for the Next Doll' you are, because the cast gets whittled down pretty brutally in those regional trials. You've got Maya Vance, the protagonist, who's this scrappy underdog from the fringe systems. She's not the most technically gifted candidate, but her intuitive grasp of doll psychometry is unreal. Then there's Kaelen Rez, the golden boy from the Core Academies—incredibly polished, politically connected, and kinda insufferable until you get his backstory about familial debt.
A lot of readers fixate on the Administrator, this semi-mythical figure running the selection process. We only get his narration in the interlude chapters, and his motives are super ambiguous. Is he trying to save the Doll lineage or end it? The dynamic between Maya and her assigned practice doll, an older model called 'Whisper', is the core of the book for me. Their silent conversations carry more weight than all the flashy trial sequences.