The Hulk has dissociative identity disorder due to repressing his emotions and many traumatic memories from the constant childhood beatings and hatred from his father, which combined with the uncontrolled mutation of his biology. Because of the nature of his powers, when he switches his personalities, his physical form also changes with his mental state. The most prominent personalities are:
- Bruce Banner: The "default" human personality, Bruce is a brilliant and emotionally repressed man with no superpowers, though he keeps himself in shape these days.
- The Savage Hulk: The most iconic and recognizable Hulk, he is green, dumb, and (naturally) communicates in Hulk Speak. This personality always wants to be left alone, in the sense of being recurrently viciously attacked without understanding why, but also always wants friends and to be loved, a conflict that fuels many of his adventures. Because he is so quick to anger he is often regarded as the most powerful of the core personalities. Generally thought to be Bruce's inner child. Curiously, he is a Gentle Giant and The Berserker simultaneously.
- Joe Fixit: The "Grey Hulk", and supposedly the first Hulk to ever appear (in terms of Marvel continuity; originally, this was the result of a coloring error and inconsistent characterization), but really quite different in displayed personality. Unlike the Savage Hulk, Joe transforms at night, or more rarely when Bruce "lets him out". He has an average intellect, and is particularly known to enjoy luxury, rich food, tailor-made clothes, pleasurable company etc. He is also the most amoral of the main personalities, having worked as a mob enforcer. Lacking the full raw strength of the Savage Hulk but being notably more crafty, he not only relies on his fists, but also employs manipulation and other underhanded tactics to gain an advantage over his enemies. Thought to represent the typical teenager that Bruce never let himself become. His portrayal is generally somewhere between a Boisterous Bruiser and a truly amoral being. Not to be confused with Mr. Fixit. Joe is the opposite.
- The Merged Hulk: Variously described as a merger of the above three personalities or a representation of Banner's ideal version of himself, the Merged Hulk has Bruce's intellect, the Savage Hulk's power, and the Grey Hulk's craftiness. He is often known as "Professor Hulk". Unlike other Hulks though, he loses his strength as he grows angrier. Too angry, and his body reverts back to Banner, while his mind regresses into the Savage Hulk. This is due to a self-imposed psychic fail-safe to ensure that he won't cause damage like he did when he lost control and killed the Leader. He's typically relegated to the role of the Genius Bruiser. He is also perhaps the most handsome of all the Hulks, since he shares Banner's face.
- The Green Scar: A personality developed during Planet Hulk and World War Hulk (see below). Intelligent (although not on the level of Banner), rational and unquestionably the Hulk's strongest physical form. He barely won against The Sentry at full power during World War Hulk. During the Heart Of The Monster storyline, he was somewhere between this personality and Savage, and became so enraged that he could barely move without causing entire continents to undergo tectonic shifts—a state which Hulk calls the "Worldbreaker".
- Doc Green or the Omega Hulk: The result of Banner's brain being enhanced with Extremis, "Doc Green" — who refuses to be known as either the Hulk or Bruce Banner — is a hyper-intelligent, arrogant pragmatist who views Banner as his "captor" and believes his actions to be morally correct because of his genius. Following his quest to "cure" all other Gamma mutates of their powers, Doc Green eventually "died" when he allowed the Extremis to wear off, causing him to revert to the Savage Hulk, as he feared becoming the Maestro.
- The Immortal Hulk or Devil Hulk: A cunning, cold-blooded figure of brutal vengeance, attuned to magical thinking and the world of gods and demons. Something of a Satanic Archetype, with the potential to be either accuser or adversary; fiercely protective of Banner despite his viciousness, and willing to destroy the world itself to protect Bruce. Like the Gray Hulk, the night is his time, and he transforms whether Banner is alive or not.