Decarabia is a Persona of the Fool Arcana, and you'll need to fuse one if you intend on filling out the Compendium in Persona 3 Reload. Even if you choose not to use Decarabia in your battles, it's a useful Persona to "unlock" since it can be used in various dryad fusions. When you first start out in your Persona 3 Reload playthrough, Decarabia's fusion won't be accessible. You'll need a special key item that's found on the Tziah Block in Tartarus.