Calvin is a Spirit/ghost of unknown origin, his Primary form is A green ghost cat but his Solid form is a Cat made of semisolid Goo/Slime (he is best described as being like a water wiggler toy in terms of his structure interior and exterior)

His outer layer is hard like Gelatin but firm and hard to push through, his inner layer is soft and thick Liquid slime that is Similar to the consistency of soap , calvin can envelop and trap others inside of him if he desires and if they are not let out it will be the same as drowning as his entire body inside is liquid.

Calvin can produce or reduce his Goo to make himself bigger or smaller at will. As a feral he prefers to be a giant Slime cat in solid form.

In ghost form calvin can physically be touched by others if he allows it, but he prefers his Goo form.

Calvins origin is unknown as he claims its not important. He likes being a ghost anyway!

Calvin is mischievous and Competitive but a loyal friend and will be there to help get a little petty revenge if needed, he loves to "play" with others by trapping them inside of him if they seem to hint at curiosity of it or ask for it.

calvins favorite form is a giant slime cat when he is in "feral" universe
his favorite form in anthro is just a standard anthro cat made of slime or his normal ghost form.