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 a demon 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.