The title 'darkest game' makes me think you're asking about 'The Darkest Game' by Joseph Schneider? That's a detective novel, a later book in the 'Tully Jarsdel' series. If that's it, the absolute key character is Detective Tully Jarsdel himself. He's a former academic in Classics, which gives him this unique, sometimes awkward perspective on grisly Hollywood murders. He's paired with his more traditional, street-smart partner, Detective Morales. Their dynamic is central.
Then you have the victim, a film historian, and the suspects from the old Hollywood scene. The killer's identity is, of course, pivotal but revealing that would be a major spoiler. The book really revolves around Jarsdel's internal conflict between his intellectual nature and the brutality of his job, so in a way, his own psyche is a key character too. The plot leans heavily on his specific way of thinking.