Kaoru Mizusaki is a supporting character who serves as a soldier in the Earth's United Forces. She holds the rank of lieutenant colonel and is the vice-captain of the assault ship Wadatsumi, where she acts as the direct aide to her commanding officer, Captain Darzana Magbaredge. Following the transfer of the Wadatsumi's crew to the battleship Deucalion, Mizusaki continues in her role as the executive officer and second-in-command.

In terms of personality, Mizusaki is depicted as a practical, logical, and highly competent officer. She possesses a calm and composed demeanor, which proves valuable during military operations and high-stress situations. She is characterized by a tendency to speak her mind clearly and directly, valuing honesty and straightforwardness. However, this trait frequently manifests as a lack of tact, leading her to be blunt or unable to read the mood of a situation. Her captain, Darzana, often points out that this behavior, alongside her defensive and safety-oriented mindset, is the primary reason for her lack of success in romantic relationships, a recurring source of gentle teasing between the two women.

Mizusaki's motivations are closely tied to her role as a military professional. She is dedicated to her duties and shows a strong sense of responsibility, often taking command of the ship as an acting captain when Darzana is absent. Her thinking is described as defensive and analytical, which sometimes causes her to misinterpret social cues or advice regarding personal matters. While she often questions Darzana's decisions from a logical standpoint, she remains a loyal and reliable subordinate.

Her most significant relationship is with her superior officer, Darzana Magbaredge. Their dynamic goes beyond a typical command structure, displaying a close-knit and familiar rapport. Darzana frequently lectures Mizusaki on her social failings, framing them as lessons on why she is unpopular, a running joke that highlights Mizusaki's naivete regarding interpersonal relationships. Despite these critiques, Darzana trusts Mizusaki implicitly with the ship and its crew. Mizusaki also interacts with the younger crew members; on one occasion, she joins a conversation with the female crew members in the cafeteria, showing an interest in their personal lives, though her attempts often yield no useful information. She is present during strategic discussions involving the prodigy pilot Inaho Kaizuka, acting as a referee during a chess game between him and the captain and asking if he wishes to be removed from the front lines, demonstrating her pragmatic concern for the crew.

Mizusaki undergoes little personal transformation throughout the series, remaining a stable and consistent presence on the bridge. However, there are subtle indications of reflection; during a moment when she is left behind while Darzana undertakes a separate mission, she briefly allows her stoic expression to falter, hinting at unspoken feelings about her role. She survives the conflict until the end, appearing in the epilogue during the ceremony for the new Princess of Vers.

Regarding her abilities, Mizusaki is a capable leader and military tactician. Her calmness under fire and defensive-minded strategies are assets in battle. While she is not a frontline pilot, her authority and competence allow her to manage the operations of a highly advanced warship effectively, a responsibility she carries out with consistent professionalism.