Do all the visual novels, including this, belong to one universe? Is there any more information on the girl who kills Seiichi in the trust-neither end?
Koko/Reiko/Void of Desire and the upcoming visual novel belong to the same world. And if you mean the little girl who appears in one of the bad endings, I'm afraid not, there is no more information on that character.
Looking forward for the new visual novel! Will it be from perspective from any known character, by the way? And for Void of Desire, I do see the story of Koko meeting Seiichi in the epilogue, but is there any information on her mom?
The new visual novel will feature new characters. Although there may be references to previous ones, it is too early to say for sure (and not to make spoilers). About Koko's mother, I think there was a line of dialogue from Lilieth talking about her. But I can't tell you from memory exactly where.
