Akaibo, more commonly known as Akago, is a significant antagonist in the narrative. His name is the Japanese word for infant or baby, and in the official English adaptation, he is simply called the Infant. He takes the physical form of a very young, pale-skinned baby with white or light lilac hair and large, empty-looking eyes. Despite his appearance, he is not a human child but a yōkai, or demon, created as the seventh detachment of the primary antagonist, Naraku. Akago serves a unique and vital function as Naraku's physical heart. By removing his heart and placing it within this separate being, Naraku achieved a form of immortality; his body could be completely destroyed, but as long as Akago survived, he could fully regenerate.

Akago was born in a sacred location known as Mount Hakurei, a place that possessed a powerful holy barrier. Although he was never directly exposed to the barrier, the purity of this environment influenced his creation, which is reflected in his unnaturally pale and clean appearance, a stark contrast to Naraku's darker, more monstrous look. This holy origin granted him a strong resistance to sacred powers, which later enabled him to survive an attack that would have destroyed a normal demon.

In terms of personality, Akago is calm, intelligent, and speaks fluently, which is deeply unsettling given his infantile form. He possesses the ability to read the minds of others and has a particular power to latch onto the darkness or resentment hidden within a person's heart, twisting their emotions to control them. He is also a schemer who, like his "siblings" Kagura and Hakudōshi, eventually grows tired of being a mere extension of Naraku. He rebels against his creator, driven by a desire for freedom and independence. To this end, he seeks to increase his own power, collecting Shikon Jewel shards and absorbing other yōkai to strengthen his position.

Akago's role in the story begins when Naraku removes his heart to become a full demon. He is placed under the care of a human woman who is manipulated into believing he is her child. During a confrontation, a monk named Master Shinsen manages to cut Akago in half. However, due to his holy origins, he does not die. Instead, each half survives, resulting in the creation of two separate beings: the half cared for by Naraku's detachment Kanna remains the infant Akago, while the other half, held by Kagura, grows into a new, more powerful incarnation named Hakudōshi. Hakudōshi acts as a separate entity, but his life remains tied to Akago, as his heart is still within the infant.

To further protect his vulnerable heart, Naraku has Akago placed inside the body of a powerful, soulless golem named Mōryōmaru. From this "castle," Akago continues his rebellion, using Mōryōmaru's body to absorb demons and gain strength, hoping to eventually overthrow Naraku. His primary relationships are defined by these connections. He is Naraku's creator and master, but he seeks to betray him. He is the "brother" of other detachments, including Kanna, who protects him, and Kagura, who he tries to recruit to his cause. He is the source of life for Hakudōshi, making their bond unique.

Akago's development follows a path from a protected secret to an active, rebellious player in the conflict. His ultimate goal of defeating Naraku fails when his creator reveals he anticipated the betrayal. In a final confrontation, Naraku allows Mōryōmaru to absorb him, only to use this as a ploy to devour the golem from the inside, finally reclaiming and reabsorbing Akago. His notable abilities include mind-reading, emotional manipulation, the generation of a powerful barrier, and, while housed within Mōryōmaru, the ability to absorb other demons and gain their powers.