I think that it might be Kokorogi or Yuuchi's step mother - or both. Or hey, it can be his mother (that gave birth to him).
We know that his step mother was a prostitute - so his step mother could meet Kokorogi. We also know that the gang, that pretended to be a part of the Tomodachi Game Management, was called by a woman. Then we know that THEY know (the TG Management) about Yuuichi's past - like a lot - so someone from Yuuichi's past might be the culprit.
We also "know" that Yuuichi killed 3 people, one of them was his teacher/father. It's been hinted on that the other 2 were his mother and the step mother but we don't know that for sure yet.
And I thinkt that, the culprit being Yuuschi's mother or step mother, is a good plot twist (cause let's face it, Kokorogi was suspicious from the beginning...).
I personally want it to be Kokorogi, cuz I really don’t like her, and I hope she disappears, but I feel like it might be Shibe, cuz I think they were hinting at it?
Like, when the game lady said they’d be better off without him, or the whole thing with him being excluded from the suspects list simply because he’s dumb and can’t hold his mouth, or when he’s hugging Kei and he’s making a sour face. But, maybe the author is purposely trying to mislead us ? Idk. Either way, I think it’s Shibe or Tenjin. Because Sawaragi knows who it is, and she’s rooting for Yuuichi, she believed that Kokorogi didn’t betray them (even tho I hope it’s her), and Kei can’t be it either, cuz she told him about it. (But maybe it’s old Yuuichi, who sometimes pops up? Idk lol), It’s so nerve wracking, I can’t wait for the next update.
Also, I’m living for the translator ranting about Kokorogi, cuz same.