Louie is a black mouse with white paws, large round ears, and a big, dark gray nose. He has several wrinkles around his eyes, indicating his old age. Later on in the show, his nose becomes black.
Louie seems to be patient and very calm, as shown in "The Man," where he endures Richard shouting at him, and even preventing him from seeing Granny Jojo. Later on, he even tries to settle an argument with Richard "by sitting down, and having a calm, reasonable conversation."