Uncle Dudley was one of Captain Marvel's many allies. He did not have powers, but did occasionally dress up in an "Uncle Marvel" costume. He managed to trick enemies into defeating themselves, rather than defeating them. His precise relationship with Billy has always been somewhat sketchy.
This is his third animated appearance. He appeared in The Kid Super Power Hour with Shazam!, and an evil version named Uncle Super appeared in Justice League: Crisis on Two Earths.