Hisui Hatsushiba (初柴ヒスイ,Hatsushiba Hisui?) is the niece of Mikado Sanzenin and also Nagi's childhood friend. She is one of the candidates for inheriting the Sanzenin fortune, though Mikado himself doesn't want her to inherit it.
Hisui has black hair, which she usually keeps tied into a ponytail with a red ribbion. She has red eyes and has a scar over her right eye. She is usually seen wearing a white dress with a black singlet underneath.
Hisui first appears when she is seen greeting her maid Yozora. Hisui asks her if Mikado intended to make Nagi his successor once again. Yozora confirms to her that he would if Nagi acquired a King's Jewel. Hisui remarks about the value of the stone before Yozora remarks that Mikado didn't want to give her the inheritance. Hisui begins to wonder if Mikado hated her, to which Yozora reassures her that he didn't hate her but everyone was afraid of her. Hisui then suggests that they both speak with him.
Sometime later, Hisui and Yozora arrive at Mikado's castle unannounced. Upon finding Mikado inside the control room for the Sanzenin assets, Hisui asks him why he didn't welcome her, before noticing Yozora's reaction to Hayate's presence. Hisui asks her if she knew him, to which Yozora cheerfully informs her that he is Nagi's butler.
Hisui is very greedy and will stop at nothing until she gets what she wants.