The killer was Mifune, the cameraman. He actually was Kira's secret boyfriend, and had planned the séance in order to have revenge and to manipulate the situation so Kira's lifeless body, which was probably hidden in the mansion, would be found. He had always thought that Kira, an excellent and careful driver, would have not died in a car accident, and since the "accident" took place near the mansion, he believed the people who had been there with her were related to her mysterious death.
Kira's murderer was Reiki Hirasaka, who was a very heavy drinker due to depression. They were in a party in the mansion and at some point he got just too drunk. When Kira tried to calm him down he threw her to the floor violently, and she died by a hitting her head and suffering a fatal concussion. He then set up everything to make people believe she had died in an accident instead, hiding her body in the mansion.
Mifune had started to target the people who were known to have been with Kira the day she died. He first attacked Sawanami, being the one with the lowest guard. In regards to Hirasaka, Mifune killed him by poisoning wine; the proof of it were some wine stains on his shirt, hidden under his tie. Hr also wanted to kill Honogi first, but then changed his mind and killed Shouko before him...
Finally, there were two "reasons" why Shouko Utakura was killed.
- First: Mifune thought Shouko was the girl who was in the party that ended with Kira's death (Sawanami had mentioned a female, but didn't say her name)... but it wasn't her. It actually was Maho Izumi, the cosplayer. She didn't say it due to the trauma she had gone through when the kind-hearted Kira, who befriended her and saved her from ending up as a hikikomori (house recluse), had been murdered. She also ended up explaining to Mifune the reasons why she commited the mistake that would get her killed...
- Second: Shouko found Sawanami's lifeless body and, thanks to the kana he spelled in his own blood, deduced that Mifune was the one who attacked him; she later commited the mistake of confronting Mifune and saying she knew the truth, but would cover up for him. Shouko was in love with Mifune and truly wanted to assure him that she wouldn't reveal his guilt, and not to mention she had decided to not tell him that she loved him since she knew he still loved Kira. When they spoke, however, Mifune misunderstood her and thought she wanted to blackmail him, so as soon as he could he strangled the girl to silence her. The reason why Shouko didn't fight back and let Mifune kill her was because she fell into despair when he attacked her.