Sakaido serves as the primary persona adopted by Akihito Narihisago when diving into id wells, digital manifestations of a serial killer's unconscious mind. This persona embodies a brilliant detective singularly focused on solving the mystery of a girl named Kaeru's death within each well. His existence arises directly from the psychological trauma Narihisago endured following his daughter Muku's murder and his wife's subsequent suicide. These events drove Narihisago to kill his daughter's murderer, resulting in his imprisonment and qualifying him to enter id wells, as only those who have taken a life can navigate these mental spaces. Upon entering an id well, Narihisago sheds all memories of his real-world identity and experiences, effectively becoming the blank-slate detective Sakaido for the duration of the dive. This transformation creates a distinct personality separate from Narihisago's burdened self.

Visually, Sakaido manifests as a younger version of Akihito Narihisago, characterized by light pink hair styled upwards and green eyes. His consistent attire includes a long brown trench coat, a lighter shirt underneath, a black vest, large flared black shorts, black leggings, trainers, and a notably long yellow scarf. This specific appearance remains constant across entries into various id wells, serving as his recognizable form within these unconscious landscapes. The disassembled state of his left arm within Tamotsu Fukuda's id well represents a unique physical alteration experienced during one specific investigation.

Personality-wise, Sakaido exhibits a calm and proactive mindset, maintaining impressive composure even when confronted with disturbing situations like dismembered bodies or distorted environments. While not immune to initial panic, he quickly regains focus and takes logical action. His defining characteristic is brilliant deductive reasoning. He approaches investigations methodically, focusing intensely on individual clues and prioritizing solving the mystery above his own survival. Despite this analytical drive, Sakaido possesses significant empathy. He views people encountered within the id wells as individuals rather than mere clues, sometimes deviating from his investigation to save someone in immediate danger. This empathy is shown when he cries over Kaeru's body in Nahosi Inami's id well, overwhelmed by the pointlessness of her death and his inability to save her.

His core function within every id well investigation is identical: to solve the mystery of Kaeru's death. Kaeru appears deceased at the start of every dive, her method of murder varying depending on the id well's owner. This variation provides Sakaido with crucial clues about the real-world killer's motives and methods. Kaeru's presence acts as both a grounding mechanism, reminding Sakaido of his detective role, and a symbolic representation of the killer's victims. Later revelations identify Kaeru as Kiki Asukai, a telepath whose traumatic experiences and unique abilities are intrinsically linked to the Mizuhanome system's functionality and John Walker's manipulations.

Sakaido's investigations contribute directly to identifying serial killers in the real world. The clues he uncovers and deductions he makes within the id well are interpreted and acted upon by the Kura support team, leading to arrests. He occasionally collaborates with other "brilliant detective" personas, such as Anaido (the persona of serial killer Tamotsu Fukuda) and Hijiriido (the persona of Koharu Hondomachi after she gains the ability to dive). His confrontations with the recurring and enigmatic figure John Walker (manifesting within wells as the arrogant detective Uraido) become central to uncovering a larger conspiracy involving the creation of serial killers.

A significant aspect of Sakaido's existence is the fundamental disconnect between his persona and Akihito Narihisago's real-world identity. Narihisago grapples with severe guilt, trauma, and a grudge against serial killers, occasionally using insights gained from id wells to psychologically manipulate them into suicide. Sakaido, however, operates free from this baggage during dives, embodying the focused and empathetic detective Narihisago struggles to be outside the wells. Over time, Narihisago is forced to confront his past and the nature of his actions, seeking to reconcile his broken self with the heroic detective persona he becomes within the id wells. This journey involves bridging the gap between his desire for redemption and his darker impulses.