Kei Yamaji (山地 ケイ Yamaji Kei?) is the 16-years-old daughter of Tetsuzan Yamaji who becomes Princess Ninja Emiha (姫忍 恵美破 Himenin Emiha?) and the deuteragonist of Sekai Ninja Sen Jiraiya. When she dresses as Emiha to assist Jiraiya, she wears a white ninja armor similar to Jiraiya's. Her special technique, the Cherry Blossoms Rain Ninja Art (忍法花吹雪 Ninpō Hanafubuki?), allows her to put enemies to sleep with a rain of white cherry blossoms. Her other technique, the Face Copying Ninja Art (忍法顔うつし Ninpō Kao Utsushi?), allows her to create face masks of other people to disguise herself.
She was portrayed by Megumi Sekiguchi.
After Toha was cursed by Chang Kung-Fu into losing his fighting spirit, Kei donned the Emiha armor to go out and fight Chang in his stead. During the confrontation however, Emiha and Manabu were both captured by Chang and the Sorcerers Clan. Fortunately, it was at that moment that Toha regained his spirit and became Jiraiya once more to save his sister. After being freed by Jiraiya, Emiha took on Benikiba while Jiraiya fought Chang Kung-Fu. After Jiraiya defeated Chang, Benikiba and the Sorcerers ninjas retreated and Kei returned home alongside Toha and Manabu.
She often teases her adoptive brother Toha, leaving most of the housework and chores to him, but cares deeply about him.