Zanni
The Zanni are often referred to as "servant" characters, but this is not entirely correct -- some like Capitano are rarely shown as actual servants, and almost all zanni have traditional uses for filling out other types of careers ranging from shopkeepers to politicians. However, they are most commonly shown to be servants of the vecchi or innamorati, and so the association is not unfounded.
The origin of the term zanni indicates that rather, they are meant to represent commoners or the everyman, for is from a dialectal version of the name Gianni, equivalent in English to "Jack." A now obsolete term, Jack-Pudding, seems to have been related; it referred specifically to street actors. Notably, The Oxford English Dictionary gives the following citation from 1648: '...The Junto-men, the Hocus-Pocusses, the State-Mountebanks, with their Zanyes and Jack-puddings!' Another English word, zany, referring to "crazy" or "wacky" behavior, came from the Italian zanni. Zanni is used as both the plural and the singular form of the word.
As previously indicated, the zanni are the most versatile of the commedia characters; they can be working for (or sometimes as) the innamorati, or may be helping the vecchi, or may have their own motivations apart from any sides.
Some stories even omit the other character types altogether and focus squarely on the zanni, as with The Marvellous Malady of Harlequin in which the full story seems to be about Harlequin becoming pregnant and giving birth to children. The Punch and Judy story, which too evolved from commedia, also focuses on the adventures of a zanni. This phenomenon is much more prevalent from the 18th century onward as the innamorati declined in popularity.
Another common description for the zanni is that they are the "masked characters," though again this is not entirely correct, as there are several zanni -- such as Pedrolino/Pierrot, Scaramouch, Mezzetin, as well as the female soubrettes -- who have as much tradition of being unmasked. The main trait common to all zanni, is they are supposed to be entertaining. This is most often achieved though comical behaviors and speeches, but can also be done through other methods like acrobatics or dancing. There are records which seem to indicate that some troupes would keep actors who were not particularly talented comedians or actors but who could pull off impressive physical feats, such as one 18th century Dutch illustration showing a company's Scaramouch tightrope-walking while playing a violin which he holds behind his head -- meanwhile the show's Harlequin is left to provide a more humorous narration over the action.
The zanni are generally supposed to be low-class characters, at least in behavior if not always in their active social status (as far as rank and riches, zanni are even known to fill out the roles of royalty, as in Harlequin Premier.) While they might sometimes be of a higher social class, they are never classy per se -- at heart they always remain cunning conmen or dull-witted country boys. In older shows they spoke in folksy dialects, sometimes with a specific region's speech assigned to them. The scripts for Turandot and Bragatto offer (translated) examples of how the zanni's speech differs from the vecchi and others. Their costumes, too, were originally meant to represent low-class forms of dress: Harlequin's colorful suit evolved from an outfit meant to portray ragged clothes that were heavily patched, and the baggy ensembles worn by Pedrolino, Pulcinella and others were meant to resemble the frumpy clothing worn by rural workmen of the Renaissance.
Around the end of the 17th century, the zanni started to be depicted with a little more elegance, and they increasingly began to have romantic storylines that rivaled (or even replaced) those of the innamorati. Pantomime took off, and the zanni came to be portrayed as magical dancers instead of bumbling servants. By the 19th century, popular shows often concerned a love triangle between zanni (typically Harlequin competing with someone for the love of Columbine.) Sentimentality, as the fashion of the day, continued to overtake them and deaden their humor; by the early 20th century the zanni had become "cute" instead of comical, visual appeal being their only purpose; and the shows they appeared in were often just plain weird.
In the second part of the 20th century, efforts were made to revive the original Renaissance methods for commedia dell'arte shows; and so in modern productions it is more typical to see the zanni as crude and boisterous, befitting the name zany, as they were originally depicted.