Ethan Sawyer was Temperance Brennan's grad school friend and the murder victim in The Past in the Present. Bones was unofficially consulting with Ethan on a previous case concerning Christopher Pelant, however apparently Ethan was in a mental institution at the time of the previous victim's death. His exact psychosis was unspecified, but one 'symptom' included the belief that Brennan's daughter was some kind of parasite that was a 'threat' to her mother.
His body was found drugged and eaten alive by wolves at a wildlife refuge, with Pelant staging the crime and leaving faked digital evidence to suggest that Brennan killed him due to his threats to Christine. He leaves a triangular clue to Bones and the Jeffersonian within his institution room, written in his own saliva as an unidentifiable code, found by a black light.