Originally, Takaki is supposed to be alive. He is Yabu and Yuto's muse. But it was around Halloween, and a prompt grabbed me:

"But... You're dead."
"Huh? I was wondering why everyone's so scared to see me."

So there. Takaki's dead. Murdered, had an accident (that Yabu knew but kept secret?), idk how he died but that's what our Yuto is going to find out. At least until Yabu cracks and he confesses everything he knew.