Vault Guardsman

Michael O'Brien was a sergeant of the NYPD, and the brother of engineer Kevin O'Brien, who became the Guardsman and accidentally died while fighting Iron Man (who was in secret Michael's boss, Anthony Stark). Although the police investigation exonerated Iron Man, and a fellow officer who was involved in the investigation tried to convince Michael it was an accident, he believed it was a cover up.

He confronted Stark at a charity party for the Iron Man Foundation, accusing him of having killed Kevin. Stark offered to talk it out, but O'Brien refused. He also attempted to question Iron Man (secretly Happy Hogan, who had filled in for his employer at a charity function) when he was in a hospital after a fight with Red Ghost's Super Apes. He also approached Roxanne Gilbert and continued to harass Stark himself. He hired a criminal P.I. whom he had caught in the act to infiltrate Stark's Long Island plant. He was infuriated when the P.I. could not turn up any evidence to support his theory