Generally, (when I do buy adoptables-I'm just now buying one, never really saw the use before) I like adoptables that are really creative. Everyone and their mother can come up with a fox, rabbit, cat, etc. character but it's rare to see someone create a human (or looks that way) with a really, really cool design. This one adoptable I'm buying looks like a human male with an rpg-esque design based off of a monarch butterfly and the creator gave each of his designs an rpg job (Jester, Soothsayer, etc.) and then gave them really cool outfits
The only reason I'm buying this particular adoptable is because, well, I just saw him and suddenly knew he would fit incredibly well for my story as my Fae Queen's jester and close confidant.
So, long story short (sorry for the huge paragraph): make the character unique but easy to fit in another person's story/OC realm/whatever