Iori Watanuki is the chief engineer of the submarine I-401, maintaining its engines and energy systems. Her technical expertise allows her to effectively manage the advanced Fog technology integrated into the vessel. She demonstrates significant proficiency in repairing and interfacing with the submarine's systems, occasionally receiving protection from energy fluctuations via a Klein Field projected by Iona, reflecting their professional rapport.

Ranking fifth in her year at the Japanese Maritime Institute of Technology, Iori studied alongside crew members Gunzou Chihaya, Sou Oribe, and Kyouhei Kashihara. Her hands-on experience aboard I-401 positions her among the most knowledgeable humans regarding Fleet of Fog technology.

Iori possesses orange hair styled in two untidy pigtails, with spiky bangs framing her brown eyes. She typically wears practical work attire: a white tank top, green cargo pants, a tool belt, brown gloves, and two-tone work boots.

Exhibiting a cheerful and outgoing demeanor, Iori frequently engages in light teasing of crewmates like Shizuka Hazumi and Gunzou Chihaya. Despite this playful behavior, she maintains focused professionalism during operational duties.