Rough origin thoughts
I was thinking a little more about Jesper and his origins...
So, we've pretty much solidified, to my knowledge, that Jesper had shitty previous owners. How his bag comes into Yarrow's possession is still in the air, I believe.
I'm not entirely sure what his previous owner did, possibly working with alchemy or natural medicines, etc., something along those lines. (?) Something that would warrant having an assistant. I do not see this person having the patience required for dealing with Jesper, and tried to make the best of it by making an attempt to cater to Jesper's strengths. Which, I guess would be "entertainment". Except, he's not naturally gifted with anything. No ability to play music, juggle, tell jokes, or do anything that could be considered festive and fun.
I get the sense that imps come in a few different forms. "Domestic" imps would typically provide some form of assistance, and "wild" imps would be ... I dunno. Living in clusters causing chaos. Probably floating and levitating and being cool, devious and chaotic...
Anyway, to my point. I could see Jesper having been basically saddled as a jester to entertain, but he just has no natural ability for these things. So, whatever happened, however it went down, Yarrow becomes in possession of the bag that Jesper inhibits and he gets to live his silly life however he wants!
I think most imps are not as... Huh, what's the word. Clumsy and silly as Jesper. Most would still be mischievous, maniacal and chaotic, but since Jesper is more of an assistant, I view him being a bit more tame and loyal. But he still fights those impulses nonstop.
Given a crumb of patience, Jesper could be such a good assistant!!! Yarrow would have to really go over some basic herbology or some shit to really get it in his head what he's looking for (if we're talking about plants here for example), but by god, he'd find it if you sent him out for it. Something a lil more stressful like potion making would be a bit nerve wracking and he will be wanting to lick, drink and eat everything that goes through those grubby little hands but he will fight those impulses!!! He can do it!!! I just don't see his previous owners having the patience. Which, to be fair, if you're after an assistant, I get not wanting to jump through hoops to make it manageable lol.
Speaking of wild vs domestic imps, I could see them come in varying shapes and sizes---not unlike humans, relatively within the same few inches of difference in height, and varying body shapes. I like to imagine Jesper is a bit rounder than usual. Dense boy.