Frisk when they get a body of their own in events smells like gravedirts. It's just a faint persistent thing, mixed in with that crushed-plant scent of golden flowers. It can't really be gotten rid of, but they don't mind too much

it's a minor minor minor con with the pro of actually being able to things without having to ask every time

they also probably smell like grass stains and smoke from 2am cookings because the castle doesn't have microwaves

and sometimes it's just fun to feed random things into a furnace

Chara smells like pondwater lol since that's effectively what they use to wash in

they share the smoke (and blood) scent with Frisk

if they've been practicing they smell just a little of magic, which is hard to describe but basically how red smells

there's probably also a bit of mothballs and dust because of sleeping in various abandoned castle rooms

and both have varying levels of bo