The Mermen of Mu, as they were nicknamed by the press, were actually the elite Royal Guard of the King of Mu. They were tasked to accompany him any where and protect him from any dangers. This proved difficult as Seamaster, the codename of the King of Mu, suffered from wanderlust and spent his time travelling the seas of the Earth in search of adventure.
The Royal Guard has existed since ancient times and were members of an aquatic race. Its membership has varied throughout the ages. However, the last known Royal Guard of Mu perished alongside their King on August 9, 1945, in an atomic bomb blast in Japan.