Carol

From Homestar Runner Wiki

Carol is Marzipan's guitar, on which she plays syrupy songs. In the Powered by The Cheat music video "Oh Yeah" from the email marzipan, it is called "Carl." Carol features a sticker of a "Don't Tread on Me"-style snake from the American revolutionary war. In monster truck, Strong Mad was going to smash it over Hometsar's head.

According to Marzipan on the strongbad_email.exe DVD, Carol is not her real guitar. Her real guitar is named Joan. However, in these commentaries, Missy, as Marzipan, takes the rare position that the Homestar Runner characters are actors, and that the events we see in their cartoons are performed for us, and not, as everything else would imply, their actual "lives." So quite how literally the "Joan" comment can be taken is debatable.