John Walker started out as Super-Patriot. He challenged Captain America, he thought Cap was too out-of-touch with America. He then replaced Steve Rogers as Captain America for a short time. He gradually overcame his grudge with Rogers, and became a member of the Avengers.
This is an ancient custom of mine, completed years ago. It was a no-brainer to turn the ML Ultimate Cap in this, the face always looked more like Walker than Rogers to me. I scraped off some of the chain-mail on the chest, and hand-made the head-wings and the star on his back. Added some rubber to make the buccaneer boots. The belt buckle is from the box set Green Goblin. I kept the shoulder stars 'cause they look cool.
As an aside, I recently bought the new Epic Heroes U.S.Agent ML. Not impressed. My custom remains on display...

Thanks for your time.

TommyX