She was adopted by Dan shortly before he blew up the animal shelter across the street from his apartment in Dan Vs. The Animal Shelter. Mr. Mumbles is an ideal companion for Dan and is often there for him after coming home from having a bad day (which is mostly everyday). Often times, Mr. Mumbles' presence can help calm Dan down and, at times, reduce the severity of whatever diabolic plan he has for sweet revenge. Dan is very protective of her, as seen in The Catburgular, as well as other episodes. He probably likes the cat more than Chris.
She plays a major role in The Catburglar (episode) when she is kidnapped, but is rescued by Dan. On several occasions, she has been used by Dan in his revenge schemes and willingly aided him. When she assists, he will usually disguise her accordingly, such as with a ski mask or a cape when they became superheroes (calling her Mumblecat).