Jiruo is a young man who serves as an instructor and leader for the apprentice Delvers at the Belchero Orphanage in the town of Orth. He holds the rank of Moon Whistle, a testament to his skill and experience as a Delver, which authorizes him to undertake deeper dives and formally teach younger explorers. Jiruo is characterized by his serious, stern, and responsible nature, which often comes across as cold or strict, particularly when enforcing rules and maintaining discipline among the children in his care. However, this rigid exterior masks a deep-seated sense of duty and a genuine concern for the safety and well-being of the orphans, something he prioritizes above all else. He is also highly intuitive and perceptive, able to understand the unspoken intentions of those around him, especially regarding his student Riko.

Jiruo's background is deeply tied to the legendary White Whistle Delver, Lyza, the Annihilator. As a child and a mere Red Whistle himself, Jiruo was Lyza's apprentice. Before she embarked on her final, irreversible dive into the Abyss, Lyza entrusted him with a crucial secret: to watch over her infant daughter, Riko, and protect her identity as the child of a White Whistle. This responsibility shapes his entire relationship with Riko upon whom he acts as a strict, caring, and sometimes overly protective guardian and mentor. Despite his frequent scolding of her impulsive behavior, he quietly supports her growth as a Delver. He also has a notable history with Ozen, the Immovable, who once offered to take him on as her apprentice, an offer Jiruo respectfully declined, stating a preference for the Lord of Annihilation, a choice that Ozen still references by calling him "brat" but without genuine malice.

Within the story of Orth, Jiruo's primary role is as an authority figure and teacher, responsible for organizing expeditions and instructing young orphans like Riko, Nat, and Shiggy in the dangerous craft of delving. He is a pivotal figure in Riko's early life. When he perceives that Riko is determined to venture into the Abyss's depths to find her mother, he makes the difficult decision to not obstruct her. Instead, he offers his silent support, leaving her a letter that gives her a head start before he and another Delver, Hablog, stage a token pursuit that ultimately allows the children to escape. This act demonstrates his profound understanding of her desires and his respect for her journey, putting her growth as a person above his strict adherence to rules. Later, his sense of duty is again highlighted when he accompanies the ailing orphan Kiyui and the nurse Mio to a caravan ship and then to Ozen's Seeker Camp to investigate the mysterious Birthday-Death Disease plaguing the town's children.

Jiruo undergoes subtle but significant development through his relationships. His early dynamic with Riko, defined by his role as her protector and disciplinarian, evolves into one of quiet mutual respect as he ultimately honors her ambition over his own fears for her safety. His past as Lyza's apprentice informs his present, and his interactions with Ozen reveal a man capable of earning the trust of even the most formidable and dangerous Delvers, despite choosing his own path. While often seen lecturing, he also demonstrates a fair and insightful nature. When Riko asks about her mother, he gives an honest, unflattering, yet clearly fond description of Lyza as a heavy-drinking, fight-picking, unbalanced individual who was nonetheless a legendary and brilliant Delver. In addition to his strong sense of responsibility, Jiruo possesses notable abilities as a capable organizer and a skilled, knowledgeable explorer whose competence is respected by veteran Delvers including Ozen.