Marie was ranked #1 popular (even ahead of Peacock) in a Japan poll according to Cyberfront (Japan SG Publisher) awhile back when the game launched in consoles over there despite being unplayable in contrast to the IGG polls where Japan was no where near the majority percentage of backers.
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It can theoretically be any character though since we need more to go on besides the character theme to be sure. I see clues/trends though however that may favor certain SG character odds. Sion who was the
main character of
Melty Blood series who originated from a different series as well didn't get her Melty theme in UNiB.
Sion Melty theme:
Encount
Instead she got a remix of the opening song of Melty Blood: Actress Again
Sion UNiB theme:
Blood Drain- Again (Hmm...
.Legend of The Skull Heart -again- both tracks are openings...
)
Blood Drain was the
opening track for
Melty Blood: Actress Again that wasn't necessarily directly related to Sion either so theoretically
Legend of The Skull Heart -Again- can simply be the theme to represent the Skullgirls cast like Sion did for
Melty Blood.
Its not just Melty Blood either, another recently announced UNiB guest character named Akatsuki who was the
main character from
Akatsuki Blitzkampf the WWII Nazi era fighting game appeared as well who had his UNiB theme recently announced called
War Start -Again-
You guessed it... it was from the
opening track of
Akatsuki Blitzkampf
War Start is the
opening of the game and not directly related to Akatsuki since he has his own theme in the game.
Akatsuki theme in
Akatsuki Blitzkampf: Awakening
Just like
Legend of the Skull Heart was the
opening track that wasn't directly related to a specific character although it
leans towards Marie who got remixes of the opening track called
Skull Heart Arrhythmia (Marie Boss stage) and
Destiny in Her Hands (Rooftops Assault Stage) where Marie is present carrying a freaking building in the background.
This also indirectly hints that the Skullgirls opening track is going to sound fairly different in the UNiB remix if you compare both the
Blood Drain and
War Start tracks and it can mean any Skullgirls character could be the guest character although with what we have currently right now I think it would lean towards Marie (Fits the overall atmosphere of the game) or Filia (Keeping up with the
main character guest character pattern).
Thats the end of my little theory which by no means is confirmed facts.