Peewit is a character created by Peyo, who appeared with the Smurfs in their movie, The Smurfs and the Magic Flute, and their 80's cartoon series. He is Sir Johan's sidekick.
When not counting Gargamel and Scruple, Peewit is one of the most commonly seen humans in the Smurf series.
In one episode, Scruple comments that Peewit is no older than he is, meaning that Peewit and Scruple are about the same age (Some might estimate them to be around 9-11, as Scruple is a mere kid in the eyes of many). It can also be assumed that Brenda and Denisa are the same age as the both of them.
His character in the cartoon show was voiced by Frank Welker. In the United States English dub of the unconnected movie, he was voiced by Cam Clarke.
In the Asterix comics, the village minstral Assurancetourix (in the English version of the comics he's called "Cacophonix") has his personality and bad musical qualities loosly based on Peewit's. Assurancetourix however is a grown man who simply dismisses the others' negative feedback as being "Misunderstood". Similar to Harmony Smurf he sees himself as the only one with true musical skills.
Peewit is referred to as William in the United Kingdom English dub of "The Smurfs And The Magic Flute".