Little is known of the early life of Miya Shimada.  It is known that she was born in California in the 1920's, the daughter of first-generation immigrants to the United States from Japan.  At some point in the 1930's, she became disenchanted with like in  the United States and deeply affected by anti-Japanese sentiment in the years leading up to World War II.   She left the U.S. and spent years "studying" in Japan, tho it is not clear what these studies entailed.  It is known by the early 1940's, she had been fully indoctrinated in the anti-Western sentiments of the Japanese government and returned to the U.S. in 1942 as a Japanese agent.

Her first action was to be involved in the capture of Army Intelligence officer Rick Cannon whom she took as a prize back to her home community in Santa Barbara.  She taunted her fellow nissei - American born children of Japanese parents - to reject American prejudice and serve the agenda of the Japanese emperor.  She revealed her new identity as Tsunami just as the arrival of Starman and Liberty Belle brought them there in pursuit of the captured Cannon.  The heroes engaged in a brief melee, finding Tsunami enormously strong and resilient and at its peak, she hurled a stack of large crates at them, which was intercepted by an elderly Japanese who caught the brunt of the impact.  Recognizing the victim, Tsunami fled and has he faded, the old man revealed himself as Tsunami's father.

Returning to her base of operations on a Japanese sub off-shore, she discovers that other members of the All-Star Squadron - Johnny Quick and Neptune Perkins - had been captured.  After Tsunami's attempts to convince the commander that they be returned to Japan for study, Starman, Firebrand and Liberty Belle arrive to free their comrades and in the ensuing battle, the sub is destroyed.  As Johnny and Belle rise to the surface and cling to a buoy for safety, Tsunami rises from the depths to attack them. Creating a massive wave to smash them, she is stymied by an energy field from Starman's gravity rod and outnumbered, she retreats below the waves.  The heroes return to shore where Tsunami resolves to attack them with an even great wave but as it forms, she is yanked below by Neptune Perkins who struggles with her.  She finds herself attracted to him but castigating herself, flees to return to Japan (All-Star Squadron #33-35).