Doolittle is not so much a character as he is a vehicle which Shaw manipulates for his own dramatic purposes. Through Doolittle, Shaw is able to make many satirical thrusts at middle-class morality and to make additional comments on class distinctions and on class manners. (It is especially witty when Eliza points out to Higgins that the Professor's so-called equality in the way he treats people shows that he has the same manners as her father because Doolittle makes no class distinctions either: the analogy wounds Higgins because he has to acknowledge that it is essentially true.)
As his name readily suggests, Doolittle does as little as possible to get through life. He is a dustman because that is easier for him than "real work." (A dustman was a person who simply collected the ashes that people put out; by Shaw's time, refuse was added to the ashes, making Doolittle essentially a garbage collector.)
The comedy connected with Doolittle is his transformation during the course of the play. Whereas his daughter wants to become a member of the respectable middle class, Doolittle is delighted that his job as dustman is so low on the social class scale that it has absolutely no morals connected to it; therefore, he is not subjected to "dreadful" middle-class morality — at least not until the last act.
When we first meet Doolittle, he comes to Professor Higgins' house in the hypocritical role of the "virtuous father" in order to rescue his "compromised daughter." It is soon discovered, however, that he threw his daughter out into the streets to earn her own living over two years ago, and, furthermore, he was never married to Eliza's mother. In fact, the people in the neighborhood won't even let Doolittle have any of Eliza's belongings. When the ruse of the virtuous father fails, Doolittle quickly changes his pitch and becomes the ingratiating pimp as he tries to sell his own daughter to the men for almost any price they are willing to pay. Higgins and Pickering are not taken in by his nauseating suggestions, however, but they are delighted by Doolittle's poetic use of the English language, by his use of rhetoric that could only be used by a Welshman, and by his ingenuity as he tries one method after another until he assumes a philosophical pose; in his resourceful rhetoric, he stoutly proclaims that too much charity has been directed at the "deserving poor." Now is it time for him to claim his equal share as a member of the "undeserving poor." An undeserving poor man, according to Doolittle, has as much right to go on a drunken binge as does a deserving poor man; furthermore, if they will give him some money, he will promise to spend it all on a drunken binge immediately and will thus be broke and ready for work on Monday morning.
The originality of this idea, and the audacity and impudence with which it is put forward, cause Higgins and Pickering to yield to Doolittle's request, and they even offer him ten pounds, but Doolittle refuses because it would involve him in responsibilities; he can't drink up ten pounds in the weekend, but he can drink up five pounds.
In the last act, Doolittle's character does not essentially change. It is only that through a large sum of money, he has been forced to accept responsibilities that he would rather not have been faced with. The immoral blackmailer and pimp of the second act has now been forced into the role of a lecturer on moral reforms, and he must now adopt middle-class morality. Since Shaw philosophically wanted to do completely away with the lower class, he is pleased to force Doolittle into accepting a position where he will not be comfortable being one of the "undeserving poor"; Shaw undoubtedly was secretly delighted at the discomfiture that Doolittle was undergoing.