Seamaster, whose only given name was Ra'a, was a founding member of the superhero team the Knights in 1940. He remained active on the team and fought alongside them throughout World War II, until his death in 1945.

Seamaster claimed to be Te Ari'ikinui, the King of Mu, a sunken kingdom in the Pacific Ocean, which was populated by an aquatic race of merfolk. This was never verified for the press at large, but he had high level, private meetings with most of the leaders of the nations of Earth which treated him with great deference as royalty or afforded him the equal respect of another sovereign. He was blue-skinned and seemed to possess superhuman, sea-related capabilities far beyond normal humans. He also commanded legions of blue-skinned Mermen who all seemed to be of the same aquatic race and possess the same underwater abilities as Seamaster. Seamaster served with distinction as a member of earth's premiere superhero team, the Knights, who had several undersea adventures and knew first-hand of his realm. Thus, most of the public believe he was legitimately the King of Mu, an undersea civilization of merfolk.

Seamaster was killed on August 9, 1945, in an atomic detonation over Nagasaki, Japan. He was attempting to halt the supervillain Crimson Dragon from destroying the city. His Royal Guard of Mermen also perished in the blast.

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