Jiruo, commonly referred to as Leader by the children under his care, is a Moon Whistle Delver who serves as a primary instructor and guardian at the Belchero Orphanage in the town of Orth. As a young man with white hair and bright blue eyes, he typically carries a serious and often frowning expression, which complements his stern demeanor. He is approximately twenty years old during the events of the story.

Jiruo’s personality is defined by his serious and strict nature, which can sometimes be mistaken for coldness. This behavior is not born of malice but from a deep-seated commitment to his duty of ensuring the safety and proper upbringing of the apprentice Delvers in his charge. Despite his harsh exterior, he is highly intuitive and insightful, often perceiving the true intentions and feelings of those around him. He supports those in his care in his own subdued manner, such as when he grants Riko permission to view her mother’s sealed letter or silently allows her and Reg to depart into the Abyss after giving them a head start.

Before becoming the orphanage’s leader, Jiruo was the apprentice of the legendary White Whistle Delver, Lyza the Annihilator. At the time of her final descent into the Abyss, Lyza entrusted him with the crucial task of watching over her newborn daughter, Riko, and protecting her identity as the child of a White Whistle. This responsibility forms the foundation of his relationship with Riko, acting as a stern mentor who frequently reprimands her impulsive nature but ultimately provides her with honest advice and silent support. He is the one who reveals to Riko the truth about her birth in the Abyss and explains that her poor eyesight is a symptom of the Curse of the Abyss rather than a natural condition. He also demonstrates a perceptiveness regarding Reg, strongly implying that he has deduced much about the boy’s origin and connection to Riko from the very beginning.

Jiruo’s primary role in the story is that of an authority figure and a source of exposition in the town of Orth. He presides over briefings for the apprentice Delvers, enforces the orphanage’s rules regarding the retrieval of Artifacts, and serves as a connection to the older generation of Delvers. After Riko and Reg’s departure, his role expands to involve investigating the mysterious Birthday-Death Disease alongside an orphanage worker named Mio. He takes the sick child Kiyui to a caravan fleet and later travels to the Seeker Camp to consult with Ozen, the Immovable Sovereign, demonstrating the trust and complex relationship between them. While Ozen once invited Jiruo to become her apprentice, he declined the offer, preferring to forge his own path, yet he remains a figure she respects and confides in.

Throughout the narrative, Jiruo’s development is subtle but significant. He transitions from a strict, rule-enforcing instructor to a figure who actively uses his authority to aid the protagonists in his own way. He leaves a letter for Riko and Reg, informing them that he knows of their plan to descend into the Abyss and that he will give them a head start before pursuing them, an act that is both a final test of their resolve and a tacit blessing. This reveals a deep, paternal care beneath his gruff exterior, showing that he honors his promise to Lyza not by holding Riko back, but by allowing her to follow her own path. As a Moon Whistle Delver, Jiruo possesses extensive knowledge of the Abyss and the skills necessary to navigate its upper layers, qualifying him to train novices, and his authority at the orphanage grants him significant influence over the lives of the young Delvers of Orth.