This is my proposed theoretical resolution to the case of Haneda Kouji, one of the greatest mysteries in the series that is involved with several unknown identities at present.
First off, Kouji met with Amanda and Asaka to have a chat in Kouji's room about some random topics. Asaka is originally Amanda's bodyguard, because, Amanda had acquaintances with the FBI and the CIA, so protection was necessary. She hired Asaka at one point from the past and nobody knew Asaka's background. I guess Asaka was a secret agent from some organization back then, with unknown whereabouts. In the conversation, Amanda ordered room service, like sweets and tea to Kouji's room. Rum was outside, observing the situation and waiting for a chance to kill all three.
Second, after the conversation, Amanda was tired so she went back to her room to rest. Asaka was with Kouji, maybe talking about the Shogi tournament the next day, and some security stories regarding Amanda that Asaka was concerned about. When Amanda went to her room, Rum, previously snuck into her room, ambushed Amanda, forced her with APTX and killed her easily. That explains why Amanda's room was clean. Rum had to act quickly, since there were 2 other alive, Asaka and Amanda, to clear up all evidences. Right when Asaka opened Kouji's room and went back to Amanda's room, Rum pushed Asaka in the room, kicked shut the door and knocked out Asaka. Seeing Kouji panicked and about to call the reception, he left Asaka unconscious there, and went ahead to subdue Kouji. Kouji managed to put up some fight with Rum (defensive wounds on his arms), but was soon overwhelmed. In the end, Rum managed to force Kouji to swallow APTX. Just when Kouji was about to die, Asaka launched a counterattack on Rum, injuring him somewhat. Rum fought Asaka and now Kouji's room turned into a huge mess, with broken objects like crockery, plates and cups. Asaka was again, overwhelmed by Rum. Just when Asaka was about to be forced to swallow APTX, Rum heard the sounds of people from neighboring rooms come back after dinner or something. (The incident happened with no witnesses, neighboring hotel residents all went out for dinner, etc so Rum took advantage of the situation to act). Asaka was still resisting and so it was too late for Rum to kill Asaka. Fearing that Amanda's death might be exposed soon enough, Rum knocked down Asaka unconscious again and fled the scene, with the thought that Asaka would be framed for killing Kouji and Amanda. Kouji still had his last breathe to go into the bathroom, filled the sink with water to cut the mirror, craft the dying message, PUT ON MASCARA, and finally died. Asaka regained consciousness, fled the scene aftera while, but people knew that Asaka fled (on the website by the uploader of the case, it says that people hear Amanda call out the name "Asaka"), but they didn't know Rum or any suspicious person fled before Asaka. In the process, Asaka took the U shard from the dying message Kouji made with a pair of scissors, but became the prime suspect for both murders.
Facts and evidences we have about this case:
*Dying message, correctly deciphered as ASACA Rum.
*The U shard is missing from "PTON MASCARA" part
*Tap water was left running in Kouji's room
*Kouji's room was very messy while Amanda's was very tidy: Crockery, broken hand-mirror, cups and plates smashed to pieces in Kouji's room.
*Amanda met with Kouji to have a chat, before she was killed
*Defensive wounds were found on Kouji's arms
*Gaito Hotta believes Asaka is female and Asaka was seen holding the hand-mirror.
*Kouji ended up with the hand-mirror to craft the dying message.
*There was a person who kept uploading the case online when the case was taken down every time
*Bruise on Kouji's palm when he held the scissors tightly
*Unknown object in the middle of Kouji's right palm
*Asaka's hiring was done in secret. No one close to Amanda knew how Asaka was hired.
*Tsutomu's body was never found and he was missing still.
So that's how I recontructed the Kouji case. To answer some questions, here are my thoughts:
I think Tsutomu Akai (missing) was an investigator of the case after it happened, and so his investigation caught BO (Black Organization)'s attention, and went missing afterwards. Asaka is in fact Wakasa Rumi, who is still keeping the U shard, and the missing bodyguard from 17 years ago. Asaka picture seems to fit Rumi (slim figure) and 17 years ago, Rumi was 20. I believe in Hotta's statement as well. Since Rumi was a secret agent (follow up from the thought above), she could pretend to look older back then, for Amanda to hire her. But 20 looks fitting to the picture of Asaka back then. As for who Rum is, I think Rum hasn't appeared in person before. I don't think Kouji knows Rum, so, his dying message most likely meant to point to Rumi (died before creating the letter "I") You might say, that a dying message of the victim always points to the culprit, exposing them. But since Kouji didn't even know Rum at all (not to mention Rum could have put on a disguise in the murder), he could only help the police's investigation by directing it to Asaka, the survivor of the case (I think he saw Rumi alive, but unable to move temporarily due to Rum's beatings).
I tried my best to simplify this big mystery, so I could be so wrong and the case could've been much more complicated than what I think. Feel free to give your opinions. Thanks for reading!