"Mike certainly has the look of a squirrel, but not the corresponding manner. Squirrels generally move jerkily, as if they walked constantly on molten lava and were afraid to make more than momentary contact. Mike moves with the easy grace of a burly policeman—the composure of a man who could kill another with his bare hands but doesn't sense any particular need to."
(source)
The easiest way to describe Mike is to say he's a born uncle. Lord knows if he had any family during his life, but he's certainly making up for it in Purgatory. He especially looks after Dolly, who trusts him absolutely, and Eric... oh, but that's another story.
Mike may be boisterous and a little coarse, but his heart is in the right place. Dolly would be lost without him, Star thinks he's God, and Lily actually holds intelligent conversations with him. He's pretty good at handling Angelo's bouts of deep depression, and he keeps Joe out of serious trouble by involving him in play-arguments.
His relationship with Ruby is unique; he confides his troubles in her, but she doesn't find him tiresome and whiney. Ruby sees something that no one else seems to: Mike's inner hero. The squirrel does extraordinarily well under pressure, rising to the occasion without fail—although usually with a bit of good-natured grumbling and a hearty bout of cursing.
"You wouldn't understand, Angelo. He... he shines. You're bound to see it someday." -Ruby