Shuichi - Probably out trick or treating just to keep an eye on Kokichi and Kaede (dressed as a skeleton).
Kaede - would dress as a princess and probably be one of the most eager people to go out and get free sweets.
Rantaro - sitting inside and watching A Nightmare Before Christmas, not dressing up as anything other than a self proclaimed cool guy.
Kirumi - the peacekeeper dressed up as a witch. She'd hand out the sweets and when the trick or treaters got home she's help them organise their stash.
Ryoma - would probably be with Rantaro or Kirumi and people would always comment about how he looks like a gremlin despite not dressing up.
Angie - she personally doesn't celebrate, but that doesn't mean she can't get sugar high in her room and scream out of the window to scare passing kids (she made one dude cry before quickly promising them that Atua will protect them, to which he started crying more).
Tenko - she's probably be dressed as herself and go trick or treating, being the most dramatic of the group.
Korekiyo - pranks literally everyone. Also everyone thinks he's possessed anyway so why waste time dressing up?
Miu - she'd help Korekiyo with the pranks by making brilliant inventions.
Kokichi - oh, he would be by far the most hyped. He'd dress as a scarecrow which he had the outfit for custom tailored by Tsumugi. He'd be that guy who has a plan on how to go about collecting the most candy. He'd have backup masks so he could go to the same house numerous times and would often scam people out of their candy even with Kirumi and Shuichi watching.
Kaito - would be eating sweets outside and offering some to the passing kids while trying not to look suspicious. He'd probably be staring up at the sky, too, looking for constellations.
Maki - either with Kaito or with Angie, eating sweets and having a surprisingly good time. If she's with Angie, she'd restrain the human sugar rush and if she's with Kaito, she'd try to stop him from offering sweets to strangers.
Himiko - she'd dress up as a mage, obviously, and do tricks // spells for the passing kids. She'd usually be too tired to go trick or treating so she just would take some of Tenko's stash.
Tsumugi - She'd be hyped for the whole night, cosplaying her favourite characters and making costumes for the others. It's her night to shine, plus she steals candy from Kokichi in exchange for tailoring his costume.
Kiibo (with a thank you to @the_bleh_to_meh_blah for noticing I missed him and for giving this headcanon) - When he sees the guys with masks out so late at night he freaks out and runs to his room. Tusmugi would explain that it was a human tradition (maybe with some particularly robophobic comments from Kokichi) and he'd go outside only to get pelted with eggs.
Happy Halloween!
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