Miss Mowcher, like Mr. Dick, is living proof of that old saying that you shouldn't judge a book by its cover. When David first meets Miss Mowcher, he's staying at Yarmouth with Steerforth. Miss Mowcher is a traveling hairdresser and has family in Yarmouth, so she's in the area. Miss Mowcher is well acquainted with Steerforth and looks him up while she is in town.
While trimming and applying lotions to Steerforth's hair, Miss Mowcher rattles off a ton of gossip about all of her clients – who is bald, who dyes his mustaches, and so on. She keeps exclaiming that she is "volatile" (22.147), which means that she is flighty and changing all the time.
In the middle of Miss Mowcher's volatility, she manages to get David to confess that there is a lady in the area he has feelings about: Emily. David can have no way of knowing that this slip will have really, really serious consequences.
Miss Mowcher knows Steerforth's questionable motives and unfaithful nature really well, so she assumes that David is a naive, harmless boy who Steerforth is going to corrupt eventually. She doesn't trust Steerforth at all, but she knows that David is probably a nice boy.
When Miss Mowcher leaves David and Steerforth, Steerforth's servant, Littimer, approaches Miss Mowcher. Littimer asks Miss Mowcher to pass a love note to Emily. Thinking that it is David who wants to meet up with Emily, Miss Mowcher believes there's no harm in giving Emily the note. But the note is actually from Steerforth, arranging his first solo meeting with his future victim. Miss Mowcher would never have delivered the note if she had known which of the two men it was really from.
After Emily runs away, the whole town of Yarmouth is talking about the disaster. Miss Mowcher is so distressed by her part in it that she runs to David and explains what she has done. She also tells David that she saw the couple fleeing through the town of Norwich just the night before last. She was too late to stop them before Steerforth, Littimer, and Emily arrived in London.
David is totally surprised at the change in Miss Mowcher's manner. No longer is she "volatile." There is no trace of gossip or mean-spiritedness in her tone. She is a completely rational, down-to-earth person. Miss Mowcher explains: she is a little person. Her whole family have been little people. It is really hard for a little person to make a living in British society. She has had to adopt this entertaining, foolish manner to keep getting hired as a hairdresser in spite of her appearance.
Miss Mowcher offers to keep an eye out for Emily and to do whatever she can to help find her. Miss Mowcher wants to make amends for her part in Emily's seduction. She realizes at once that David doesn't trust her, and demands: "Trust me no more, but trust me no less, than you would a full-sized woman" (32.81). David feels bad and agrees.
Later on, we find out that Miss Mowcher has carried out her promise to get revenge on Littimer for deceiving her and using Miss Mowcher to get to Emily. When David and Traddles visit Mr. Creakle's prison and find Littimer captive there, the warden tells them that Littimer was finally arrested for robbing one of his employers. The person who caught him was none other than Miss Mowcher, who saw him on the docks attempting to leave London and held onto him physically until the police could come and arrest him. And Miss Mowcher gets her just reward for her sense of justice: she is "highly complimented by the [judge], and cheered [by the crowd] right home to her lodgings" (61.100).