This interesting-looking starfish guy is actually a demon. According to the Megamitensei Wikia, Decarabia is the sixty-ninth spirit listed in the Goetia (which refers to a practice which includes the invocation of angels or the evocation of demons, according to Wikipedia).
Something of interest is that, like Ose, he has legions of demons under his control. Here we see more use of the term legion, which probably explains the Persona’s inclusion in the Fool Arcana. Decarabia appears to be a starfish but will sometimes take the form of a man if his conjurer (basically a magician) instructs him to.
There’s not much else on him, but he apparently originates from the writings in The Lesser King of Solomon - one of the most popular books of demonology.
I’ve noticed that a majority of Persona in the Fool Arcana are demons with the exception of Orpheus. Maybe demons are just very fitting for this Arcana since they usually cause chaos - a major part of the Fool Arcana. I’ve no idea.
As always, your opinions & comments are always welcome!