Historically: Lady Kasuga, born Ofuku, was the daughter of Saito Toshimitsu and was raised by her mother's relatives when her father died in 1582. She married Inaba Masanari. She was later employed by Tokugawa Ieyasu as the wet nurse of his grandson, Takechiyo. When Takechiyo became Tokugawa Iemitsu, the third Tokugawa shogun, in 1623, Ofuku was promoted to the title of "Tsubone", or "Court Lady", and controlled the shogun's harem of Edo Castle. She became the power behind the shogun and a major influence on his policies of Isolationalism and anti-Christianity.

In Mirage of Blaze: When Yuzuru says that he is feeling unwell, Takaya jokes that he's been possessed by the spirit of Kasuga no Tsubone.