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I have compared the two

Leave this for debate here....
The ears maybe look a little like each other at this angle, its hard to tell as the Mystery Man is one Peach of a Angle, you could not ask for a more Ambiguous Shot with Specific Lighting.
Hair seems a bit more curly than Bichir's its lighter too but that can be put down to lighting.... Hair matches Rigby's color and his is curly a bit but then the Mystery Man seems to have more short hair at the bottom than Rigby but the image is so hard to make out and Rigbys hair could be combed/slicked back so that it looks like this?
Here is best back shot i could get that shows how his hair can look short from behind.

Sorry for low Resolution and here is sharpest image from the side

Its still hard to make out....
I think only way is to try and measure the proportions and how his head features line up..... size of jaw in relation to forehead in relation to ears etc etc.
To me the Hair does not 100% Match Bichir.....
But who knows with the lighting... i think its hard to judge on clothes too as Birchir seems to be wearing a more dark blue/green but again the shots of him are in dark and cant get a good resolution to make a comparison.
The Jacket and Trousers in color and style are a 100% Match for Rigby though...
Here is a High Res Image
Bichirs is similar color to Daniels.


So the Mystery Deepens.....
When trying to RULE OUT RIGBY
We have to consider the following...
1)This shot would have to be after Hell Breaks out, i cant see any reason for Daniels to have her Gun out and look worried unless they have encountered a Xenomorph or Ultramorph or something else... regardless of which... Rigby gets infected and if they saw a Xeno first or something else then surely they would had dealt with him better as far as Quarantine etc.
Logically this has to be after the Back Burster and Rule out Rigby.
2) Height... Waterstone is 5ft10 " Bichir is the same and so is Franco, and the imagine in question while Daniels is a bit in the foreground and looks a inch or so smaller, her knees seem more bent... i would assume these are similar size which is why i wondered if it could be Franco.
Benjamin Rigby looks a good 6ft 1" in the Last Supper shot but its hard to tell as they are all not standing the same distance from the Camera.
MY CONCLUSION?
It looks like Rigby from behind, a lot points to it being him apart from Height and also well surely this happens after Hell breaks out and we have to assume Rigby is the first casualty
I have completely ruled everyone else out.... unless
*Ridley Scott has Wardrobe Malfunctions.
*Crew leave and depart twice and thus change clothes.
*Franco recovers and has his hair cut shorter and his stumble trimmed down etc.
This only leaves (despite looking like Rigby) BICHIR (despite clothing different color)
The only other Option would be Matt Scaletti
Hair texture is a match, height is a match, ears look similar but i have not managed to find a from behind shot of him, he does have some Moles/Beauty Spots on his cheek near eye and near his eye and on his neck to the right side.
But the Mystery Man Angle would hide these...
I find no reason for why there should even be Scaletti not mentioned in the Last Supper Crew or even shown in the Trailer at all...
Unless...
1) He was awoken after as a rescue crew..
2) Was part of a previous crew...
The latter would maybe fit Daniels as she finds the Dog Tags but this could be explained by the crew splitting up at some point too.
The only other Explanation for Scaletti or a yet unnamed cast member would be this...

Remember this shot with the Code?
There was only One Logical way this worked out, and its like over a 3 day Period.. and seems to hint that another crew went down first...