From Homestar Runner Wiki
Cardboard Homestar is Homestar Runner's replacement while he is "on vacation." However, the real Homestar was later revealed to have been locked in the closet. Cardboard Homestar is manipulated by Marzipan, whom he repeatedly tells is "so smart". He is the counterpart to Cardboard Marzipan, with whom he has a conversation in an Easter egg in Quality Time.
Cardboard Homestar also attends a choir with Marzipan, as shown in Where U Goin' 2?
A precursor to Cardboard Homestar can be seen in Strong Bad's character video.