Dejah Thoris originated in the works of Edgar Rice Burroughs as the love interest of John Carter, being the titular princess of Burroughs' original Barsoom novel, A Princess of Mars. Both characters were introduced in this book, serialised in 1912 and released in collected form in 1917; as such, they are in the public domain. The same is not true of handmaidens Brin and Loo, who originated in a modern continuation.
