Momoko Kouzuki is a C-Class Arm on Mermaid Island and the partner of Aoi Miyasato. She is an Extar, meaning she carries the A-Virus and can transform her body into a weapon when aroused. She has shoulder-length brown hair tied back with a bow, and she is often seen with a sleepy expression marked by half-closed eyes and puckered lips. Her figure is noticeably plumper compared to other characters, a trait that sets her physical appearance apart.

In terms of personality, Momoko is remarkably laid-back and easygoing, a quality she shares with her Liberator, Aoi Miyasato. She has a playful tendency to shirk her duties, often doing so in a jocular manner. For instance, when called to report by Captain Kasumi, she answered the call but jokingly denied being present. Despite this casual attitude, she is not defiant; she follows orders without complaint and dutifully reports when summoned, showing that her laziness is more of a humorous facade than a true refusal to cooperate.

Her motivation appears to be maintaining a relaxed and carefree existence on the island, rather than pursuing any larger personal goal. She does not seem driven by ambition or conflict, instead content to go along with whatever situation arises.

In the story, Momoko and Aoi are the first Extar and partner duo that the protagonist Mamori encounters after arriving on Mermaid Island. Initially, they present themselves as friendly, but they soon begin to tease and play with Mamori, deliberately trying to fluster her and lower her guard. This introduces the new arrival to the dynamics and dangers of the island in a lighthearted but effective manner.

Her role is that of a supporting character who helps establish the setting and the rules of the Extar-Liberator relationship system. Her interaction with Mamori serves as an early demonstration of how partners can use intimacy and teasing to trigger an Extar's transformation.

Momoko's key relationship is with Aoi Miyasato, her Liberator. They function as a pair, with Aoi responsible for exciting Momoko enough to trigger her Drive transformation. The two share a similarly relaxed temperament, which makes them a compatible and effective team. She also has a subordinate relationship with Captain Kasumi, to whom she reports when called.

As an Extar, Momoko's notable ability is her transformation into a gun-type Arm when she is aroused by her Liberator. In this form, Aoi can wield her as a firearm in combat. The specific details of her weapon's capabilities are not extensively detailed, but her role as a ranged weapon makes her a valuable asset in battle.

Her character development is minimal within the series. Momoko remains a consistent presence throughout her appearances, embodying the same relaxed and slightly mischievous personality from her introduction onward. She does not undergo any major personal transformation or crisis, instead serving as a stable and humorous supporting figure in the island's daily life.