Ah, that suspicious bag Sato had the teacher dispose of last chapter, I feel like it was probably Suu/the stalker being disposed of.
I feel like the best friend could be the foil to Sato.
Sure we have all these creepy ppl obsessed with Sato (and legit being a threat to her, but they were sloppy and Sato brought them all to their knees), but as of now, BFF is seemingly the most sane person of them all, as well as how she low key has a hunch and recently started to pay more attention to Sato-chan. She'll probably notice something others wouldn't ever have, with their tunnel vision. She's getting signals and hints that somewhere's off with her, and that's probably enough, as it's all slowly building up. It feels like one day, she'll see that one big "DANGER" sign, (whether Sato realises she's noticed or not), and then maybe actively try harder to interfere/do something about it.
Or not. Either way, she's the only not-yandere character in Sato's life rn, and she's already wondering about the aunt/Sato's current lifestyle, heard the lolicon's accusations (she doesn't believe him, but ofc, it'll still be on her mind), aware of the existence/disappearance of Shio-chan, and now interacted more with the brother (+sees Shio-chan again. If Shio-chan keeps coming back to mind like this, BFF will really be weirded out). I don't remember who else she's talked to, or what else she's thought of, but those are already some of the main pieces. She isn't really doing anything rn (unless she continues talking to the lolicon I guess? Hearing him out?) but she's been casually observing Sato and hasn't alarmed her; that's a big enough deal I find. (Alarmed her, in the sense that Sato currently doesn't have a reason to be cautious of her, and isn't on her guard/seeking to terminate her, so BFF can continue unknowingly poking around)
