I think it is likely to be Klaha, though Mana's cousin did vocals for the partial reunion and did a good job. Of course everyone I think would love it to be Gackt, and who knows he may decide too for Kami.
I'm keeping a close eye on Mana's blog for news/hints of any sort of MM reunion, all three vore members have spoken with each other so that's a good sign.
Another thing about the gig, Testu was in attendance and was touched they performed one of the songs from his "era".
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