A man with a past traumatic experience developed dissociates amnesia. Every time he experiences something traumatic, his id persona hiding in his unconscious mind comes out taking over his body. This persona causes the man to transform into a powerful and unrecognizable being.
His hair, eyes and his clothes completely change. How?
When the man wakes up to his normal self, his case of amnesia kicks in without knowing his secret ability.
When he's traumatized again, the id persona kicks in and transforms in an instant.
Id: The Id is the unconscious that is impulsive, a child-like behavior of the psyche that is connected with the pleasure principle, an instinctive drive to seek pleasure and avoid pain. It is expressed as a basic motivation force that reduces psychic tension. It is the force of our needs, desires, impulses, and particularly our sexual or aggressive drives. It is the dark inaccessible part of our personality it knows no values, judgments, good vs. evil, or no morality.
You can read up on the id, ego, and super-ego by Sigmund Freud if you want more information on it.