Jack Flag, a character that was primarily used as a supporting cast member in the Captain America universe in the early 90s, brings a unique perspective and comic relief to the Guardians of the Galaxy. During the Dan Abnett/Andy Lanning run on the series, Flag at this point in a wheelchair thanks to an injury sustained at the hands of Bullseye of the Thunderbolts is rescued from the Negative Zone and his spine is repaired by the medical team at Knowhere (a floating celestial observatory the Guardians use as a home base). At the urging of Star-Lord he sticks around, despite the fact that he hates and doesnt understand all things cosmic. Flags distaste for all things cosmic could potentially bring some clever meta-commentary to a movie franchise. Plus having a regular ol human being on a team that also features a talking raccoon and a tree is easy comedy that writes itself.