• His standard casing is made of a magically synthesized plastic polymer - think PVC or baked Sculpey.
• The objects making up his casing may display properties uncharacteristic of the materials they're made out of. The most extreme example of this is his mouth, which he cannot remove or replicate. If he's in a setting where he expects to be touched he'll make his casing slightly softer through this ability.

That blurb isn't figurative. The guy who made him literally exchanged him for a memory. Wizards are like that.