Naoki Matsudo is a childhood friend of Kei Anno. Both enlisted in the international defense organization SPIRITS, with Naoki volunteering as a co-pilot for the giant robot Devadasy.

Within SPIRITS, Naoki is depicted as having romantic feelings for Kei. This dynamic is complicated with the arrival of another pilot, Amara Minakushi, which triggers feelings of jealousy in Naoki. She pilots the Devadasy mecha while wearing a one-piece swimsuit as part of its operational requirements.

After an incident where Kei, under the influence of nanomachines and special drugs, attempts to sexually assault Naoki, the narrative indicates she forgave him the following day.

Naoki's role expands when an alien entity captures her and forces her to pilot a hostile, biomechanical mecha. Placed in the cockpit of this enemy machine, she is confronted in combat by the Devadasy, piloted by Kei and Amara. During the confrontation, Kei maneuvers Devadasy to precisely pierce the enemy cockpit, resulting in Naoki's rescue from the alien entity.