WARNING: The following contains spoilers for Episode 29 of Yashahime: Princess Half-Demon, "The Girl Named Rion," now streaming on Crunchyroll, Funimation, Animelab, and Hulu.
When the half-demon daughters of Sesshōmaru get trapped in Mount Musubi's powerful barrier, Moroha finds herself immune to the barrier because of the spiritual energy she inherited from Kagome. Moroha is the first to discover Rion's apparition sleeping in the center of the mountain before the Dream Butterfly wakes her up. Rion allows Moroha to enter the barrier to rescue Towa and Setsuna from their illusions and they all return to where Rion awaits. After the girls introduce themselves and befriend Rion, they discover that she's the daughter of Beast King Kirinmaru.
Not only is Rion another demon princess that shocked the Great Dog Demon cousins, but she surprises them further by revealing that she actually died 600 years ago. She claims that her selfish father couldn't accept her death and wouldn't let her move on from this world to the next, binding her soul to the earthly plane. As an overprotective father who won't let Rion leave him, Kirinmaru uses the Dream Butterfly to keep her asleep so they can see each other in their dreams. This obsessive behavior causes Rion to resent her father who imprisons her with red threads of fate.
Rion tells the half-demon princesses about her father's plan to bring about the Degenerate Age that will end the world as they know it. After declaring that she's against Kirinmaru's plan, she asks the cousins to help her end her father's tyranny and prevent him from fulfilling his plan. When Setsuna suddenly sees the red threads of fate binding Rion to Kirinmaru, Rion begs Setsuna to use her Yukari no Tachikiri to sever their link. Setsuna successfully cuts down the Dream Butterfly between Rion and Kirinmaru which causes Rion's spirit to drift toward the afterlife. However, Kirinmaru instantly feels the Dream Butterfly disappearing and latches onto Rion so she can't leave. Despite his powerful hold on her, Rion reclaims her free will in an artificial body and uses the Kyūyōkon Root to undo the barrier around Mount Musubi.
After absorbing Mount Musubi's demonic barrier, the Kyūyōkon Root turns into the Star-Slicer Flute. Rion plays the flute to say goodbye to her father through a song before handing it to Towa. In exchange for the flute, Rion asks to travel with the half-demon princesses and they welcome her with open arms. When Towa finally plays the Star-Slicer Flute herself, it suddenly transforms into the legendary sword known as Zanseiken. Rion explains that Zanseiken pairs with the Bakuseiken, which is the sword that her father Kirinmaru wields. These powerful and ancient swords were once created and offered by Amatsumikaboshi, the god of the stars. Rion believes the Zanseiken may give them enough power to defeat her father once and for all.
Once Rion sets out with the three half-demon cousins, they run into Riku who was secretly protecting them from demons the entire time. Riku is shocked to see Rion again but offers to take care of her for the time being so Kirinmaru can't see the whereabouts of the three cousins. With this new demon princess at play in the fight against Kirinmaru, fans are eager to see the demon princesses banning together in an ultimate showdown.
