Riku is a major antagonist in the Inuyasha sequel series Yashahime: Princess Half-Demon. Operating under the guise of a pirate and benefactor of a demon bounty hunting business, he is secretly a servant of the series' main antagonists Kirinmaru and Zero. His true intentions unknown, Riku manipulates the series' protagonists into eliminating his enemies, all the while conspiring to take their Rainbow Pearls. As the series progresses, he starts to develop feelings for Towa Higurashi, the series' main protagonist and the daughter of Lord Sesshōmaru, causing him to take on a more anti-heroic role.
Much of Riku's past remains shrouded in mystery, with even his true nature being debatable. It is implied that he is actually a demon-puppet that was brought to life. While much of Riku's origins are unknown, it is known that he is over two hundred years old, having originally been a retainer of Lord Kirinmaru of the Dawn, the demon lord of Eastern Lands. When Kirnimaru learned that his rival Tōga, the dog-demon ruler of Western Lands, had been mortally injured in an epic battle with the demonic-dragon Ryūkotsusei, he dispatched Riku to gather medicine and treat Tōga before he could succumb to his injuries. Despite his fatal injuries however, great dog-demon had received word that his human lover, the noblewoman Lady Izayoi, and their half-demon infant, were going to be murdered by Takemaru of Setsuna, a samurai who was jealous of Izayoi's feelings for Tōga. Thus he set off to rescue them despite the severity of his wounds.
Though Riku managed to acquire the medicine needed to save Tōga's life he was unable to arrive in time to treat him before his final battle with Takemaru. This ended up resulting in Tōga's death, having sacrificed himself to ensure that Izayoi and their son, whom he named Inuyasha, could escape. Riku had arrived at the scene just as Izayoi's mansion had begun to burn to the ground, with Tōga and Takemaru having still been inside.
After this, Riku returned to inform Kirinmaru of the dog's death, as well as his lord's sister Zero. Zero, who secretly loved Tōga, was devastated upon this news, and thus made a wish upon the sacred Shiloh Jewel. Riku watched as she wished away her emotions, which resulted in the creation of the seven Rainbow Pearls. Riku was the only other person to witness this.
Sometime after these events, according to Riku, Kirinmaru decided to abandon and cast him aside, leaving him under the services of Zero. This caused Riku to develop a strong loyalty and appreciation towards her. Sometime after this though, Riku eventually started to act more on his own accord and went about his business without his masters' supervision. During this time he became a pirate, though he made the standard of never stealing from those who were women, children, or poor unfortunates. At an unknown point, he became the principal financier of Jyūbei's demon bounty hunting business, using his powers to bestow upon him much wealth.
Prior to the birth of Inuyasha and Kagome's quarter-demon daughter Moroha, Riku also warned Kagome about the dangers of the Grim Comet and that Kirinmaru and Sesshōmaru were coming to kill her daughter. He would later witness Inuyasha and Kagome's defeat at the hands of Sesshōmaru and Kirinmaru, though they managed to ensure the escape of their child.
To almost everyone he meets, Riku is shown to be nearly unfailing friendly, polite, sophisticated, gentlemanly, and jovial. This does however, make him appear goofy at times. Despite being a pirate, he shows a significant lack of greed and is completely unthreatening to those he just meets. He also seems to enjoy meeting new people and making friends. Despite this, he is a very mysterious individual, performing many contradictory actions to his stated goals and in service to many other people. This has caused a number of characters to question what his true intentions are and where his actual loyalties lie.
Beneath his friendly exterior, Riku is shown to be extremely manipulative, cunning, and deceitful. Time after time he proves himself to be a master manipulator, secretly orchestrating many of the protagonists' early battles behind the scenes all so that they can eliminate the Four Perils for him so that he may take their Rainbow Pearls. Using his skills at manipulation, he is often able to play both sides of the conflict without them even knowing, making them all his unwitting pawns. For the most part, even though he powerful enough to do it himself, Riku prefers to stick to the sidelines while watching his plans being performed and those he manipulates fight. This shows he prefers to let others do what he doesn't have to do himself.
His skills at deception and deceit are shown to even rival that of Naraku's, as he is even able to fool many intelligent and perceptive individuals, such as Kagome and Setsuna, into seeing him as an ally that can be trusted. Also like Naraku, Riku is shown to be very intelligent and knowledgeable of other demons and magical creatures, which he often seeks to use to his own gain. He is also a sadist, as seen during Homura's death where he took great pleasure in watching the mad-demon be burned alive. He also seems to enjoy the thrill of fighting, having much excitement in battling strong opponents.
Despite his secretly wicked nature, Riku does indeed possess a sense of honor. Despite being a skilled liar and manipulator, he seems to detest people going back on their word and always makes sure to fulfill his end of a bargain. He also seems to respect and sincerely reward those who serve under him, such as Jyūbei and Takechiyo. He also makes it a rule to never rob women, children, or poor individuals when he performs piracy. Despite this, he is more than happy to use loopholes in order to get what he wants.
Riku's true allegiances "seem" to lie with his madam Lady Zero, despite how much she holds him in low regard. This likely stems from her taking care of him after Kirinmaru supposedly cast him aside, causing him to view her as a mother-figure. He is shown to care a great deal about her and attempts to act as her voice of reason. He was also devastated when Zero was briefly killed by Setsuna, and chose her over Krinmaru when they were about to do battle. Despite feeling cast away by and eventually betraying Kirinmaru, Riku still respects his master. For unknown reasons, he despises the Four Perils, despite all them serving the same master as he.
Riku also seems to have a strong interest in the concept of love, often pondering about what it means to truly love someone. He seems to have a rather limited yet warped view of it, believing that to love someone means killing them. He seems to firmly believe that humans and demons loving each other will only end in pain and destruction. Despite this, he bares no prejudice towards humans, instead referring to them as beautiful creatures due to their limited lifespans. Similarly, despite disapproving of how they are brought to be, he holds no prejudice towards half-demons either.
A significant factor in Riku's development, is his growing interactions with the series' main protagonist Towa Higurashi. From the beginning, Riku takes a profound interest in Towa, going to high extents to befriend her in their first meeting, despite her initial attempts to ignore him. He is shown to respect and acknowledge her due to being Sesshōmaru's daughter and holds her in a much higher regard than the other half-demon princesses. Unlike the others, Towa is the only one Riku gives proper honorifics too, always addressing her as “Lady Towa”, and seems to almost completely overlook Setsuna and Moroha in favor of her. He also keeps an eye on her, studying the progression of her developing powers with strong interest. While it was implied that he was initially only interested in her because he believed her to be the strongest princess and so that he could manipulate her to achieve his goals, Riku eventually comes to genuinely care for Towa. This stems from not only his continuing interest in Towa's abilities, but also from being enamored by her kindness and sincerity as well as the compassionate and gentle heart she has gained from living in the modern era. Unlike others who find Towa’s modern day habits odd or annoying, Riku is amused by them, fueling his curiosity and fondness for her. This curiosity into her character and appreciation for her kindness and sincerity, eventually causes Riku to develop romantic feelings for Towa, gradually falling in love with her. As time passes, he does little to hide this fact from her, showing a charming and seductive side to him when he decided to have a "date" with her and gave a nearly full-blown confession about how he really felt about her.
Even when they briefly became enemies, Riku was willing to sacrifice his life for Towa's when Totetsu attacked them and was even willing to lie and betray his masters to help ensure her safety. He also cares for her well-being and happiness as well. When Zero asked him about how to unlock Towa's true powers, Riku attempted to feign ignorance. This was because he knew the only way to achieve this was by killing Setsuna, Towa’s beloved sister. Knowing how much Setsuna's death would devastate her, Riku attempted to discreetly safeguard her for Towa’s sake, despite the mutual dislike he feels for Setswana. This further shows his care for the half-demon princess and his regard for her feelings. As time passes, Riku’a love for Towa becomes so strong that he eventually forsakes all loyalty to both Zero and Kirinmaru, choosing to become Towa’s “bodyguard”. His love for her is so strong that is willing to sacrifice his life for her’s without hesitation and will go to any lengths to ensure her safety. Even after it was revealed that killing Kirinmaru will kill him as well, Riku considered his life of little consequence if it meant protecting Towa, further showing the extent of his love for her.
Aside from Towa, the only other person Riku is shown to be close too is Kirinmaru’s daughter, Rion. Riku seems to act as an older brother-figure for her, and is shown to be extremely protective and loyal towards her. Like with Towa, Riku cares deeply for Rion and is willing to sacrifice his life for her’s.
During a later battle with Kirinmaru, it was revealed that Riku, in truth, despises his existence due to his artificial origins, causing him to develop a somewhat nihilistic outlook on his life. Since he is an artificial demon, Riku feels that he has no real place in the world, seeing himself as only an imitation of an actual living being. It is also shown that much of both Riku’s usually happy persona and manipulative mindset are actually fronts to hide his inner sadness and misery over living what he feels to be a meaningless life. During such moments when his true origins are revealed or implied, Riku noticeably becomes much more solemn and serious, reflecting his secret unhappiness with himself. This began to change, however, after falling in love with Towa, which instilled him with purpose and allowed him to truly enjoy his existence.