She is very adorable (Just look at her) and is a very special character. No matter how many times she hurts Dan, he will never get mad at her. As even Chris, Dan's best friend, has managed to get on Dan's 'list', Mr. Mumbles is obviously extremely important to him.
She seems rather intelligent, and is rarely in danger through her own actions or Dan's constant caring for her.
It is also important to note that the few people (besides Dan) that Mr. Mumbles can stand to be with include Elise and Imposter Dan. She has been shown to, when Dan is not in the same room, to bite and claw Chris horribly, indicating that she dislikes him. She also seems to hate Ben, even jumping from Dan's arms to attack when close.