Riku emerged from Kirinmaru's left horn, shattered during a conflict with the Great Dog Demon Tōga six centuries before the series. Crafted as a demonic incarnation, he was nurtured by Kirinmaru's sister Zero and became Kirinmaru's most loyal retainer. Tasked with procuring medicinal herbs for the mortally wounded Tōga, Riku arrived too late—Tōga had already sacrificed himself protecting his human lover Izayoi and their half-demon son Inuyasha from the samurai Takemaru. Riku observed Zero channel her grief into a wish upon the Shikon Jewel, fracturing it into seven Rainbow Pearls that sealed her emotions.

Initially appearing 200 years pre-story, Riku exhibited an emotionless, simplistic persona, grappling with fundamental concepts and social subtleties. He evolved into a sophisticated gentleman with an affable, optimistic exterior that concealed his manipulative and observant core. Operating as a pirate, he adhered to a self-imposed code prohibiting theft from women, children, or the needy, while funding Jyūbei's demon-hunting enterprise through gold generated by his powers.

His primary drive involved collecting Rainbow Pearls to restore them to "madam" Zero, whom he served with fierce loyalty after Kirinmaru's alleged abandonment. He orchestrated the elimination of the Four Perils by manipulating half-demon princesses Towa, Setsuna, and Moroha, covertly securing the Pearls during clashes. He displayed sadistic tendencies, notably savoring Homura's fiery death, and espoused a twisted philosophy that loving someone required killing them. Yet he admired humans' transient existence, dubbing them "beautiful creatures."

Riku transformed profoundly through his bond with Towa Higurashi, Sesshōmaru's daughter. Initially intrigued by her heritage and abilities, he monitored her progress, criticizing her swordsmanship compared to her father's. His fascination grew into romantic attraction due to her modern-era-nurtured compassion. He prioritized her safety, sacrificing himself to shield her from Tōtetsu and betraying Kirinmaru to ensure her survival. He protected Setsuna to spare Towa grief, deceiving Zero about Setsuna's location. Post-resurrection, he declared independence from his masters, though his aims stayed elusive.

Riku bore human-like features: slightly tanned skin, auburn hair in a topknot with side bangs, green eyes, and violet eyelid stripes. His eyes shifted hue after pivotal events—purple following Kirinmaru blinding him, lavender after applying hand cream, and turquoise upon resurrection. He wore layered traditional garments: a striped blue kosode, hakama leggings, and a yellow furoshiki shoulder wrap, later replaced by a black haori. A blue earring adorned his left ear, enhanced by a sword-shaped accessory.

He commanded formidable demonic powers, recognized as exceptionally potent by allies and foes. His combat skills encompassed superhuman strength—overpowering the colossal Tōtetsu—durability, agility, masterful swordsmanship, levitation, and flight. He employed hydro-cryokinesis, freezing seawater to rescue Towa, and conjured shields from Tōga's ashes. His earring facilitated matter manipulation (transmuting substances, disintegrating enemies), teleportation, and summoning entities like boats or allies. He occasionally wielded conventional weapons, including a sword summoned via his earring or Zero's staff, which he used to encase Kirinmaru's daughter Rion in protective silk.

His ultimate objectives remained enigmatic post-resurrection. He implied that reuniting the Rainbow Pearls would trigger a "devastating Degenerate Age," allegedly Kirinmaru's scheme to reset the world. Speculation suggested Riku might seek to reshape the feudal era into a technologically advanced society mirroring the modern age, yet his true intentions—whether serving Zero, acting autonomously, or safeguarding Towa—were never clarified.