WARNING: The following contains spoilers for Episode 21 of Yashahime: Princess Half-Demon, "Secret of the Rainbow Pearls," now streaming on Crunchyroll, Funimation, Animelab and Hulu.
Episode 21 of Yashahime: Princess Half-Demon is titled "Secret of the Rainbow Pearls," but a better title might have been "Secret History of Riku." Yes, this episode reveals more about the Pearls' history, but the more interesting revelations are about what's driving the mysterious pirate Pearl collector. While some of his motivations are still mysterious, even to himself, apparently, we start to get a picture of who and what he is. As well, we start to learn just what he and Towa mean to each other.
The Rainbow Pearls are seemingly tied to the Shikon Jewel of Inuyasha fame, which at the time of this episode's prologue is possessed by Lady Zero. After seeking out the Great Dog Demon and learning that he has died, Riku returns to Lady Zero to let her know what happened. Zero seems very distraught at the news of the Dog Demon's death and starts crying. Who knows what her relationship was to him, but it was clearly deep enough to cause tremendous grief, which upsets Zero even more.
Lady Zero uses the Shikon Jewel to rid herself of both her demon power and her emotions, transforming her tears into the seven Rainbow Pearls. It's interesting to see how these Pearls are tied into Inuyasha's history, being a direct result of the Shikon Jewel and Inuyasha's birth. It's also interesting that if Lady Zero turned herself into a human here, she's still alive and causing trouble 200 years later, so clearly she regained her magic abilities at some point.
This backstory serves as a platform to tell us more about Riku. The Riku of 200 years ago is very different than the suave, cunning Riku we're used to. He frequently refers to himself as a puppet throughout this episode, which seems to mean he was created by Sesshomaru. When Lady Zero mourns the Dog Demon, Riku doesn't seem to understand what tears are, and when Lady Zero angrily tells him to use his head, he literally starts moving his head around in confusion. He has a child-like innocence to him -- perhaps he has just been created and is new to his human body.
Zero's moment of grief clearly had a strong effect on Riku. He is fascinated by Zero's "pretty" tears, and hence is fascinated by the Rainbow Pearls. Riku cares deeply for Zero because she cared for him after Kirinmaru cast him out. He even goes so far as to state that if it hadn't been for her, he'd be dead. He wants to collect the Rainbow Pearls not to gain their power but to return them to their creator, his mistress Lady Zero. He runs into a problem, however, when it comes time to take the Rainbow Pearl from Towa.
There have been hints of flirtation between Riku and Towa before, but now these flirtations are blossoming into something more. Riku admits that, even though he works for Kirinmaru, he has been unable to resist helping Towa, and that he doesn't understand if his feelings for her are love or something else. Towa also admits that she doesn't understand love, but she understands liking someone -- and she likes Riku. Riku goes on to defend her in her de-powered state from the returning Tōtetsu, taking some well-targeted poison bee stings in her place. Riku clearly impresses Towa, to the point where she willingly gifts him her Rainbow Pearl after the battle.
This frankly doesn't seem like the best idea, but Towa has never been the best strategist. She always just seems to do what she thinks is right for the people she cares for, and as she sees it, she is just returning Lady Zero's rightful property to her. She isn't too concerned about losing her power until Setsuna points out that now Kirinmaru will be stronger than ever. Towa should really start looking before she leaps, but it's worked out for Riku at least, who leaves quite happily.
On a final note, Episode 21 of Yashahime says goodbye to Tōtetsu. He was a fun, gregarious villain, but he couldn't keep his head forever. It's nice to see Moroha finally have a head to take to Jyubei as well. Maybe she can finally make a bit of a dent in her debt. Towa also learns from Kaede that Rin is her mother, but this development gets rushed through so the episode can focus more on Riku's backstory. Maybe Towa will figure out the connection between the Dream Butterfly and the Tree of Ages next time.