Which character? Geto
What do they want? Clothes (generic)!! Honestly I've been meaning to have him ask for stuff for a long time so this post is a blessing. He would request something casual and leave it up to Jesper because he's not too particular as a whole. He just wants more things to wear that aren't [generic snek meat] and [cult leader].
Can they supply materials? He will venture into the BB&B ruin as many times as necessary, but he's useless in terms of crafting so other stuff is probably a no go
What are they trading? Geto can probably offer some level of gossip but also it's funnier if he agrees to "a favor" and then Jesper makes him suffer for it.

Which character? Yu Junghyeok
What do they want? More of what Jesper already made for him. Maybe something comfy at some point?
Can they supply materials? Same as Geto, probably not
What are they trading? Junghyeok will hunt/gather and cook, he also is agreeable to manual labor depending on the task, and you can just send him on errands into the ruins for whatever Jesper may want at any point