yea, it's about bloodedge.
Though, im not sure even there is more than 1 volume, basically, Nu fuses with Ragna fake azure grimoire I think and becomes the black beast by herself.
Ragna loses all his memory and has random memories like jubei and the rabbit. Jubei is not called Jubei but has another name. Basically some other random things, there is a manga of phase zero but only 1 chapter so far, or afaik.
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Guilty Gear - (Main) - Baiken (Sub) - A.B.A - Dizzy - Anji Mito - Zappa - Sol Badguy
Blazblue - (Main) - Rachel alucard (Subs) Platinum the Trinity - Hakumen