Right I looked at the characters folder again, after ruling out every existing playable character (except OOO Purple Eyes), I'm left with the following:
Jma
Psy
Skl
Tdr
Zan
Skl for Skull, Tdr for Todoroki, Zan for Zanki, leaving us with:
Jma
Psy
While Psy might mean for Psyga, it might also stand for Purple Eyes TaToBa, though it might be under the OOO file anyway.
So question is, what does Jma mean? The only Riders starting with the letter 'J' are J and Joker. None of them can fit 'Jma' as a short term. Using it as a pronunciation doesn't seem to match any other Riders as well.