Gonzaemon Kato was the former captain of the Bentenmaru before his death and is the father of Marika Kato. In the anime, he is revealed to have faked his death and become an imperial pirate, going by the name Ironbeard.

Gonzaemon's family has a long history of service, both in the imperial navy and as pirates. Before becoming captain of the Bentenmaru, he was a naval officer.

At some point, he met and married Ririka. Sometime later, Ririka left Gonzaemon to raise their daughter, Marika.

Shortly before the start of the series, Gonzaemon reportedly passed away due to food poisoning, setting off the events that led to Marika becoming captain of the Bentenmaru.

(Source: Mouretsu Pirates Wikia)