Alfred Doolittle enters the home of Mr. Higgins after finding out that Eliza is there. At this time, Higgins and Col. Pickering are still having an avid conversation about Higgin's impressions of women and relationships, when Mrs. Pearce enters the room and announces him.
Higgins refers to Doolittle as "the blackguard" and Pickering holds doubt about that specific title. However, it is true that Doolittle is a dustman and he is described quite vividly in the play:
Alfred Doolittle is an elderly but vigorous dustman, clad in the costume of his profession, including a hat with a back brim covering his neck and shoulders. He has well marked and rather interesting features, and seems equally free from fear and conscience. He has a remarkably expressive voice, the result of a habit of giving vent to his feelings without reserve. His present pose is that of wounded honor and stern resolution.
The reason behind Doolittle's greatness as a secondary character is that he serves as an intermediary between Eliza's old life and her new beginning at Higgins's. We must keep in mind that Eliza's background has left a strong impression in her manners, speech, and behavior. All these were a direct acquisition from Doolittle.
Another great characteristic of Doolittle is that he embodies the transition occurring in Victorian lower classes. In his case, he obtains money from Higgins, after trying over and over to profit off Eliza's stay in his residence. Once Doolittle becomes "middle class" after receiving the profits, he realizes what "being of money" encompasses: With money comes the appearance of having money and living with the highest moral and social codes of conduct. This is impossible for Arthur to do, and his character sends out the message of how the lives of the Victorian middle classes are based on snobbery and futility.
This is not a far vision of Shaw's own perception of society. Doolittle is a sort of mouthpiece for the playwright. The fact that he exposes society for what it really is through Doolittle is what makes the play almost three-dimensional: We can almost hear Shaw laugh at society in plain view.