WARNING: The following contains spoilers for Episode 30 of Yashahime: Princess Half-Demon, "Hisui the Demon Slayer," now streaming on Crunchyroll, Funimation, Animelab, and Hulu.
After Sesshōmaru severs the Dream Butterfly that steals Setsuna's dreams in order to keep her mother alive, Setsuna is finally able to get some sleep after years of being awake. Setsuna tries to prevent herself from sleeping because she needs to use her valuable time to save her mother before the Silver-Scale Curse kills her. But, she ends up falling asleep from pure exhaustion anyway. Towa watches over her twin sister while she's sleeping but soon steps out to talk to Kaede. During Towa's absence, Princess Ayia steals Setsuna away in her sleep to take her to her mansion and change her into a kimono.
When Setsuna wakes up confused and disoriented, she's initially hostile toward Princess Ayia and demands to know where she is. Princess Ayia further explains that she wants Setsuna to take her lessons for her because they're the same height and age, making her the perfect double who will go unnoticed by her instructor. Given Princess Ayia's sob story about the difficulties of being a princess and political pawn, Setsuna eventually agrees to replace her for the afternoon so the princess can enjoy a rare taste of freedom. However, the etiquette instructor who shows up to conduct the lessons is none other than Hisui the demon slayer who was standing in for his older sister.
Since they were old friends who worked together with the demon slayers, Setsuna and Hisui instantly recognize each other. Hisui is surprised to hear that Sestuna felt sympathetic toward the princess and expresses his belief that her time with Towa has changed her for the better. He further explains that he's standing in for his older sister Kin'u, who inherited impressive spiritual abilities from their father and became a nun. While skipping out on conducting the actual lessons, Hisui opens up to Setsuna about his family and his motivation for becoming a demon slayer.
Hisui is the son of Sango the demon slayer and Monk Miroku who were part of the group of main characters in the original Inuyasha. He admits that his father trained and expected him to become a monk just like him, but Hisui soon lost interest in this path when his older sister Kin'u already filled this role before he could. His other older sister Gyokuto ended up taking over their mother's shop by creating and managing weapons for the demon slayers, forcing Hisui to forge his own path. Hisui later grows to admire their uncle and leader of the demon slayers, Kohaku, who risks his life to protect people. This is why he ultimately decides to join the demon slayers, where he eventually meets Setsuna for the first time.
When they discover that Princess Ayia was captured by a demon, Hisui tells Setsuna to safely stay behind and tenderly grabs her hand before asking her to have faith in him. While Princess Ayia reveals her own romantic feelings toward Hisui after he saves her, it becomes clear he doesn't reciprocate her feelings when he returns to Setsuna. Given this exciting development, fans are eager to see how Setsuna and Hisui's blossoming relationship unfolds.