After receiving a summer internship at the prestigious Kleinmann Institute straight out of high school, Keith Remsen was at the heart of an explosion at Dulles International Airport. His sister Teddy was wheelchair bound and he lay in a coma for weeks. He awoke after the White Event to discover that some shrapnel was lodged in his brain, giving him the unusual power to enter others' dreams.
Remsen was eventually drafted into the Army and used his power to counsel other paranormals who had been drafted and clear them for combat duty. During this time, he helped Jeff Walters get over the guilt of his family's death in the so-called Black Event that destroyed the city of Pittsburgh. He also entered the dreams of Dave Landers and investigated the mental block placed in Landers' mind by the psychic-powered paranormals known as the ESPeople. After some initial misunderstanding, the secretive ESPeople realized he was not a threat, and mindwiped him and left him on a country road. After an altercation with a biker gang, Keith made his way back to his army base. He later suffered severe guilt for passing an unstable paranormal, Harlan Mook as being fit for duty, only for Mook to commit many crimes including an assassination attempt on the President. Remsen eventually tracked Mook down, and shot and killed him.
Remsen was later driven insane by his own power and confined to a psychiatric ward following the war that was narrowly averted by the Star Brand.
Nightmask had the power to enter the dreams of others, and to interact with their dream selves. Originally he needed the help of his sister Teddy to return from these dreams, but later learned how to do so himself.
As his power grew, Nightmask became able to influence the dreams themselves, to project his Nightmask dream self into the real world, to create illusions, and even to travel between dimensions to some extent.