Billy Russo was good friends with Frank Castle before his family was slaughtered. Russo later went off the grid and became an assassin for hire called Paladin, and was eventually hired by AIM to work as a permanent Enforcer for them.
He was eventually sent to assassinate Captain America alongside the Winter Soldier. However, during the ensuing battle, Winter Soldier's memories ended up causing him to kill Paladin to protect Captain America, striking the beginning of his redemption.
