[Corvina]

misono isn't actively singing anymore, isn't she?
She also has MD, an illness affecting ears with hearing loss and ringing, right?
Mr. Blistah is a good guess, I think, as he came back after a hiatus in 2022 and he has several songs with Kuu (including a single and he can take over the parts of other rappers live).
Some of her Japanese fans suggested DJ Koo from TRF or some comedians (after the one she had in Universe Tour).
It's certainly someone you can distinguish by voice as they muted the video as soon as you could hear it and called the voice a spoiler.
We do not know if the person is there with her in presence or just for the interludes again as well, though she's still rehearsing rn and afaik interlude movies were done some time ago.