What are Kevin Parson's personality traits?

Kevin Parson can be described from three different angles and himself represents the mode for the triad referenced in the book's title. Kevin has a split personality, which ultimately serves to advance the theme of the text, which can be summarily described as addressing mankind's struggle between good and evil. He is the vehicle for advancing this theme in that he himself contains three personalities, with one serving each of three functions: struggle, good, and evil.
Kevin's persona functions as the intermediary between two separate identities: Sam (good) and Slater (evil). Sam is the small child that Kevin leaves behind in an abandoned warehouse; one reading could see Sam as the good that Kevin feels he's left behind in himself. Slater is the terrorizing character that makes repeated threats on Kevin's life, or perhaps the evil Kevin is trying to get away from within himself. A nuanced read might also see Kevin as the Freudian ego in flux between the id and the superego.