Brass is the son of Yuji Watanabe, the karate tutor of Dan Ketch. He is an expert martial artist because he was trained by his father as soon as he could walk as a child. Sean then joined the Navy and trained as a commando operative in the United States Navy SEALs which is where he earned the nickname "Brass".
After leaving the Navy, Sean met a girl named Theresa and fell in love with her but she was accidentally killed by a car attempting to escape an attack by the Mandarin on the Avengers. Sean learned that the Mandarin had been working with other villains as part of the "Acts of Vengeance", and swore revenge on all the villains that had taken part.
He began by attacking the Mandarin's operations in Chinatown and he ended up aiding Wolverine and Ghost Rider in rescuing his father's other pupil Jack D'auria from Deathwatch and his ninja minions. Brass lost his mutant powers after M-Day.