Bitters

Bitters is human... ish.

Well the body is human. Or at least it was. Now it's more like a prison. Fun fact, not all spirits, demons, and curses are supposed to be able to go inside people. Possession is actually a very specific skill type. One that the curse called Bitters doesn't have. It's what you call an "Obsession" not to be confused with a "Possession". Obsessions are the bad days you get some times when nothing seems to go your way. Once upon a time, your bad day was a job well done for Bitters. Well, back when Bitters didn't have a name... or a body. So how'd it come to this? To put things simply, some lines got crossed with a mortal who was trying to summon the greater spirit Bitters used to serve and instead of dealing with the trouble, the greater spirit just sent a little curse to teach the mortal a lesson.
Queue a lesser spirit that never once had an individual thought in all its existence now being trapped inside a human vessel with magic so powerful it broke the very laws that curses exist by.

Suffice to say Bitters is what you'd call a "cursed curse".