Mr. Bigs is nothing but a living comfort plushie. He's not very bright, but as long as he's making someone happy, then he's happy. His body and pawpads squeak like a dog toy when squeezed, and overall he's a lot like Baymax from Big Hero 6, just more cuddle-sized!
When he's on his own and not comforting someone, he doesn't really know what to do with himself, so will typically just sit down somewhere and let bugs crawl on him. Maybe he can provide comfort to them?
Even though he has no mouth, he can speak.