system
Given the following post, respond as if you're commenting on Reddit:
human
[Marvel] How many Hulk entities are inside of Bruce Banner? What makes each one different from the others?

So I know there's more than one, but what makes them different from each other?

Also, how did each one manifest or come about?
gpt
Well, there's the Classic/Savage Hulk - The dumbest of the personalities.  His intelligence is at best child-like.  Generally wants to be left alone.  Manifest shortly after the Grey Hulk stopped manifesting.

Grey Hulk/Joe Fixit - among the weakest of the personalities, but of average intelligence.  But has a mean streak that rivals Wolverine.  Originally appeared after the Gamma accident.  Operated on a day/night cycle, Banner during the day, Grey Hulk at night.    Manifested again much later and worked as a bouncer in a Vegas Casino under the moniker of Joe Fixit.

Devil Hulk - Never actually manifested physically, said to represent Dr. Banners darkest character traits.  His "Dark Side" if you will.

Professor Hulk - Almost as strong as Savage Hulk, and as smart as Dr. Banner, with his educational background.  For a time it was believed that Professor Hulk was all of Dr. Banner's personalities merged in to one, and that Dr. Banner was finally cured of his multiple personality disorder.

The Green Scar - Much stronger then the Savage Hulk as a byproduct of being much angrier.  This personality is a smart as Dr. Banner, but showed no evidence of Dr. Banner's educational background.  He is, however, tactically brilliant and is more than willing to work with Dr. Banner in finding solutions to the rare situation when fists is a poor choice.  This is the dominate personality when he returned to Earth after being exiled by Mr. Fantastic, Dr. Strange, Iron Man, and Black Bolt.  He and his alien army conquered much of New York City.  Turned Madison Sqaure into a gladiatorial arena.  And beat The Sentry, one of the most powerful super heroes, into submission.  Came into being when Dr. Banner attempted to use yoga and other meditation tricks to regulate his anger.  It resulted in a "blending" of Dr. Banner and Savage Hulk.  Savage Hulk displayed some of Dr. Banner's cynical humor and Dr. Banner could tap into the Hulk's strength when needed (Dr. Banner has tossed a car, and knocked The Abomination on his ass).  During his exile in space, it became it's own personality.

The World Breaker - Functionally the same as the Green Scar as far as personality traits goes, but the strongest incarnation to date of the personalities.  One step caused earthquakes across the eastern coast line of North America.  He is almost pure rage and anger.  His appearance, thankfully, have been few.