The List of Superhuman Characters who Merge Together

[Randy Jackson]

This began as a question about if Firestorm was the first superhuman to merge multiple people into one physical body, but now I'm turning it into a list. 

* I'm not counting things like combining mechs as the driver's bodies did not become one.

The list so far:

3-D Man

Agent Axis

Alloy

Brother Voodoo

Captain Marvel (Genis-Vell)

Captain Marvel (Mar-Vell)
Captain Triumph

Death's Head

Deadman
Dr. Double X
Dr. Occult

Ego
Fighting American
Firestorm

Gemini (Zodiac) 

Infinite Man

Overmind

Piccolo(DragonBall Z) 

Rebis

Starchild(Peter Wrexham) 
Starhawk

Vanguard

Ultra the Multi-Alien

I'll also be creating a list of characters who split into multiple bodies. That list is here

[Philip Portelli]

There was Doctor Occult and Rose Psychic.

And Starhawk and Aleta.

The original 3-D Man. 

[Commander Benson]

Lance Gallant and the ghost of his brother Michael merged to become Captain Triumph.

[Randy Jackson]

I figured there were other characters. 

[Dave Palmer]

Doctor Double X from Batman and Larry Trainor/Negative Man might also count as combinations of two personalities.

[ClarkKent_DC]

I'm surprised that I just thought of Billy Batson and Captain Marvel.

[Randy Jackson]

I don't remember too much about Doctor Double X, but in Negative Man's case he split from Trainor when it was time to go into action rather than merge. I think that's a different sort of idea, more like Jamie Madrox or Twice from My Hero Academia. 

Dave Palmer said:

Doctor Double X from Batman and Larry Trainor/Negative Man might also count as combinations of two personalities.

[Dave Palmer]

Merging/splitting.  Where does that put Triplicate Girl/Duo Damsel?  Are they one person or three (two) merged?  There is some evidence that each can lead a separate existence.

[Jeff of Earth-J]

ClarkKent_DC said:

I'm surprised that I just thought of Billy Batson and Captain Marvel.

I thought of Billy Batson and Captain Marvel right off, but wasn't sure if they'd count as two different personalities. Rick Jones and Mar-Vell, however, should definitely fit.

[Randy Jackson]

I would say Luornu is definitely a splitter -- although there is debate about whether her whole form is more powerful than her duplicate forms. 

Dave Palmer said:

Merging/splitting.  Where does that put Triplicate Girl/Duo Damsel?  Are they one person or three (two) merged?  There is some evidence that each can lead a separate existence.

[Randy Jackson]

I've always found it debatable about Captain Marvel. Does he switch places with Billy? Is he an older version of Billy? Is he Billy with an older body? There's evidence from the Golden Age that they do merge (Billy asks Captain Marvel for help with a college admissions test). 

As far as Rick and Mar-vell, IIRC they switched bodies, going back and forth in the Negative Zone. Rick and Genis-vell definitely merged though. 
Jeff of Earth-J said:

ClarkKent_DC said:

I'm surprised that I just thought of Billy Batson and Captain Marvel.

I thought of Billy Batson and Captain Marvel right off, but wasn't sure if they'd count as two different personalities. Rick Jones and Mar-Vell, however, should definitely fit.