From Homestar Runner Wiki
Carol is Marzipan's guitar, on which she plays syrupy songs. In the Powered by The Cheat music videos "Oh Yeah Yeah" from the email marzipan and Seasonal Sweaters from Decemberween In July, she is called "Carl". She is modeled after a nylon-string classical/Spanish guitar. Carol features a drawing of a snake. In monster truck, Strong Mad was going to smash her over Homestar's head. Marzipan played Carol in a band with The Cheat (drums) and Strong Mad (bass) in the toon Cool Things. The sound Carol makes in the song "Cool Tapes" and the short "One Two, One Two" has a Fuzztone-like distortion effect applied to it. So Carol is either an acoustic-electric guitar, or there was a microphone used in these instances so that the effect could be applied. Carol also appears when Strong Bad auditions for people to take over checking emails when he retires. Marzipan plays two scroll button songs, both of which Strong Bad dislikes. However, he later uses one of them.
According to Marzipan's comments on the email marzipan (found on the strongbad_email.exe DVD), Carol is not her real guitar; her real guitar is named Joan. However, in these commentaries, 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". It is debatable how literally the "Joan" comment can be taken.