CASE SOLVED!
Enough details have been uncovered and brought into connection to declare this case as solved. Please wait a bit for
post-game summary.During the celebration, Nitori was showing off the incredible Kappa tech, such as optical camouflage. Orin volunteered Kisume, sensing a corpse, turning her bucket invisible. As it happened, everyone saw a corpse of someone Kisume killed.
YESKisume, as usual, was doing what she does best, dropping down onto people and stuff. Today however, an accident occured, she had dropped straight onto the back of the head of Parsee. With the momentum from dropping from a really high place, she effectively killed Parsee by damaging her medulla.
Naturally, Kisume panicked. She knew she had to hide the corpse as soon as possible, but she heard that people are heading towards the bridge from underground. She quickly tore off the bottom of her robe to clean up the blood, and hid Parsee under herself in the bucket.
The group heading outside consisted of some underground youkai. Kisume was relieved when she noticed that Satori was not there, or else she would've got caught immediately. They were heading to see Nitori's demonstration of her newest inventions, and Yamame picked up Kisume and forced her to go with her. She needed Kisume as an witness to prove that she wasn't really dirtying up the rivers anymore and want to have better relationships with the Kappas.
(Most of the information above isn't necessary to solve the case, it's just me trying to explain why Kisume would go to the demonstration with a corpse under her...)
When they got there, Nitori was looking for a volunteer to test her Optical Camouflage. Orin has smelled smell of corpse coming from Kisume's bucket for a while, so she used this opportunity to force Kisume up onto stage to use the camouflage. Perhaps others, like Orin, was always curious to see what's inside the bucket, so no one really cared Kisume's objections.
When Nitori used the Optical Camouflage on Kisume, what they saw inside the bucket was the corpse of Parsee, they realised Kisume is a killer.
Good job everyone,
another one that was solved pretty quickly.