He was a part of Cap's little team that included Falcon, Nomad and Vagabond (Nomad's girlfriend). Since Steve was stripped/gave up the Captain America moniker, he was The Captain and led the group fighting criminals his own way. D-Man was the loveable lug of the group, like the Rhino when he tries to go straight.
Falcon was the vet, Nomad was the *****e and D-Man was the noble one. He was trying to learn as much as he could from Steve and become as much like him as possible. He always tried to do the right thing on his own and would always ask himself what would Steve do. When he was locked-up for running with Steve, he passed up the chance at being broken out by Sidewinder because he knew it wasn't something Steve would do. Vagabond started to fall for him a little bit and it led to a lot of friction with Nomad. He had a heart condition that he hid from the group out of fear Cap would sideline him so he couldn't help.
Steve offered him the first spot on a new Avengers team (they were broken up at the time). The very next issue though, he was blown up on a booby trapped Quinjet while helping Cap and the Flag Smasher fight ULTIMATUM troops under the influence of the Red Skull (who was in the middle of his own long awaited resurrection story).
He came back and was found by US Agent and Falcon years later in the Arctic. He was mute, living in Avengers Mansion then Bernie Rosenthal lost him in Central Park and he became the leader of a group of homeless people in Zero Town under the city. Around that time he stopped showering and became an occasional running joke.
