its probably one of her co-workers there is no way it is just for her Oshi that she took a part time job in that location.
That one coworker might be a fan of the same idol
Although, given the 'handshake events'/how easy it is to touch them in meetups and such i expect it to lead into an 'entertainment industry' arc or so (tho spica herself becoming an idol might bring too much attention as opposed to her being like a behind the scenes person or so lol)
I wanna say Lover but I feel like the twist will be it was the wife when she found out he was cheating and decided to kill everyone and set him up so he suffers for his whole life.
it says on that 'theory/evidence board' at the beginning that they split up, would be quite messed up if she killed his family for nothing but unless she didn't tell him or so, i would imagine he'd move away for his own safety for a relationship not working out
Be kinda hard for the wife to set herself on fire tho unless she hired someone or so