You know the drill comment on your prediction on who you think the murderer is!
Reading through the chapters a couples of times, I can't say that I'm entirely sure, but in mystery stories like this there should always be enough clues to figure out who the culprit is. And the culprit should probably be one of the people presented.
I don't think it's the mistress of the husband, because she has an alibi similar to the husband since they were together at the time. And Spica probably would've thought of the husband as a contributer to the murder and guilty since she touched him.
Like others have pointed out, Spica didn't include Meg-san as one of the good people. But this was 13 years ago, she'd be too young to do a murder like this I think. Though of course she can look younger than she is.
Manager likes grilling. And he's a former delinquent. But I don't necessarily think that's a reason for murder.
Tani-san is quiet and uses head phones similar to the perpetrator, but there's really no motive for him. People earlier in the thread commented that he watched idols and that Spica might've touched him and found out about him before, and jobs at the store to find the killer, but she didn't know that the husband wasn't the killer before she touched him I think? So I don't think that is possible.
Fukuda is irritated quickly because of small details. It was noted that there was trouble with the neighbors because of shouting and noise. Could she be living in the same apartment complex? And she was so mad she burned up all of them. It says that the apartment building didn't allow kerosene stoves, but maybe she stole it to burn them from the husband.
I don't think inspector Megure is the culprit. But he does have the line of if someone covered up cases like the Gemini, so maybe a hint that he covered up his own case. Having Okuma as the culprit would be doing the same twist as the previous on, so that's probably not the case, although could be a hint to this too I guess.
I'm not entirely certain, but the Meg-san comment is probably the biggest hint that she's behind it, since she wasn't included in the good people comment and Spica has most likely touched all of them at some point and knows. But I think that the culprit might also be Fukuda because of a lot of the hints might lead up to this, if she lives in the same apartment complex. But that might be a stretch I guess.