Kirinmaru is a central figure in the narrative, serving as a primary antagonist. He is known as the Beast King of the East, a daiyokai of immense power who once stood as an equal rival to Toga, the Great Dog-Demon who ruled the western lands. In a bygone era, these two great demon kings fought alongside one another to destroy a fragment of a celestial threat known as the Grim Comet. Following the death of his rival, Kirinmaru eventually entered a long period of slumber, leaving his affairs to be managed by his elder sister, Zero, and his top subordinates known as the Four Perils.

Kirinmaru possesses a complex and often contradictory personality. He is depicted as a powerful and ancient warrior who, on the surface, can appear disinterested, bored, or even casual about events happening around him. This detachment extends to his subordinates, whom he often does not bother to remember by name. Beneath this apathetic exterior, however, lies a Blood Knight who lives for the thrill of a worthy battle. He is driven by a prophecy, foretold by the Shikon Jewel, that he will be killed by a half-demon who can transcend time. This prophecy fuels his deep-seated hatred for half-demons, as it was a half-demon who killed his beloved daughter, Rion. This personal tragedy is the core motivation for his prejudice, transforming a desire for a good fight into a genuine, grief-fueled animosity toward those of mixed heritage.

Within the story, Kirinmaru is awakened from his slumber by his sister, who warns him that the daughters of Sesshomaru and the daughter of Inuyasha are the half-demons who may fulfill the prophecy. His primary goal becomes the elimination of this perceived threat. However, his actions are not always direct. He engages the three young half-demon princesses—Towa, Setsuna, and Moroha—on several occasions, easily overwhelming them to test their strength and his own fate. He also forms a tense and uneasy alliance with Sesshomaru, whose motivations are shrouded in mystery, and the two engage in battles that shake the earth and sky. In the second act, he resorts to underhanded tactics, such as stealing the Black Pearl that contains Moroha's parents, Inuyasha and Kagome, to blackmail her into retrieving a magical item called Akuru's pinwheel.

Kirinmaru's key relationships define his role in the story. His most significant bond is with his daughter, Rion, whose death is the source of his hatred for half-demons and his desire to manipulate time itself to be reunited with her. His relationship with his sister, Zero, is also crucial, as her schemes and death further propel the conflict. He is the creator of incarnations or pieces of himself, including Riku and Osamu Kirin, who are made from his broken horn and severed arm, respectively. These incarnations act independently but share a psychic link with him and are fated to perish upon his death. His rivalry with Sesshomaru is a central point of conflict, as Sesshomaru is the heir to his old rival's domain and the father of the prophesied half-demons, forcing them into an uneasy but recurring adversarial dynamic.

Throughout the series, Kirinmaru undergoes a notable development, moving from a seemingly bored and detached ancient demon to a more tragic figure. His motivations are reframed not simply as a villain's desire for power or destruction, but as a father's grief-driven quest to undo a past tragedy. In his final battle, he is confronted with his own failings as a parent and ultimately accepts defeat. He reconciles with Rion, allowing her to sever their fated bond, leading to their peaceful death together.

Kirinmaru is an extraordinarily powerful daiyokai whose abilities rival those of the Great Dog-Demon. He possesses immense demonic energy capable of altering the weather and atmosphere with his mere presence. He is a master swordsman, able to match and overpower skilled opponents like Sesshomaru, Inuyasha, and the three half-demon princesses simultaneously. His arsenal of abilities includes telekinesis, powerful energy blasts, defensive barriers that can reflect attacks, and the power of flight. He wields a sword, the Bakuseiken, which is a powerful weapon in its own right. He has also demonstrated aerokinesis, or wind-manipulation, and pyrokinesis, using a flame from his hand to incinerate magical objects. His power is such that even the combined might of the Rainbow Pearls, which were created by his sister, is no match for his own strength.