Iori Watanuki serves as the chief engineer of the submarine I-401, overseeing engine maintenance and energy systems with expertise honed through her education at the Japanese Maritime Institute of Technology. Graduating fifth in her class alongside peers Gunzou Chihaya, Sou Oribe, and Kyouhei Kashihara, she stands among the most knowledgeable humans regarding the Fleet of Fog’s advanced technology. Her orange hair falls into two unruly pigtails with spiky bangs framing brown eyes, while her practical attire—a white tank top, green cargo pants, brown gloves, work boots, and tool belt—reflects her hands-on role. Cheerful and outgoing, she frequently engages in lighthearted teasing with crewmates like Shizuka Hazumi and Gunzou, though her relationship with the latter remains strictly platonic.

Her dynamic with Iona, the Mental Model of I-401, transcends technical collaboration. Iona entrusts her with critical repairs and deploys a lesser projection to assist during maintenance, utilizing a Klein Field to protect Iori from engine-induced energy surges. This synergy highlights their mutual trust, blending professional reliance with personal camaraderie forged through shared challenges and interdependence.