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Sumika is Takeru's childhood friend. She has been his neighbor since they were little. Their rooms are right across from each other, with only a small space separating their windows, and they often have short conversations before going to bed. Sumika is madly in love with Takeru. However, her bubbly personality and rather idiotic demeanor prevents him from realizing her true feelings. She has a signature attack that she has dubbed "Drill Milky Punch".

MuvLuv Unlimited

Sumika's existence in the Unlimited universe is initially of top concern to Takeru. However, he is later told that there are no records of her ever existing and eventually forgets about her.

MuvLuv Alternative

In Alternative, the brain held within the blue container of Kasumi's room is revealed to be Sumika. Because Professor Yuuko was never able to complete her theory in Unlimited's universes, Sumika's role within Alternative IV is never realized. However, once Takeru manages to deliver Extra-Yuuko's complete theory in Alternative, Sumika becomes the most important asset in humanity's war against the BETA as the 00 Unit (Zero-zero Unit). As a humanoid supercomputer with unparalleled ESP, Sumika must be slowly "trained" by Takeru to become more humane. His initial attempts are frustrating, as Sumika's mind is completely unstable. She is only capable of incomprehensibly screaming about killing all of the BETA. Takeru manages to slowly bring back her original personality by drawing upon memories from Extra's Sumika.

Sumika's first deployment is in Operation 21 against the Sadogashima Hive. While piloting the Susano'o, Sumika manages to fire two powerful electromagnetic cannon shots against the hive, completely obliterating its Monument. However, after Takeru experiences a sudden memory recall of him having sex with Meiya, Sumika's mind overloads, and she falls into autistic mode, rendering the Susano'o completely immobile. This leads to the prioritization of her rescue and the eventual detonation of the Susano'o unit, which virtually wipes Sadogashima off the map.

After Operation 21, Sumika reveals to Takeru that she was brutally (tentacle) raped by the BETA after they captured her. Apparently, the BETA were conducting research on humanity and discovered that humans show a far more positive response to pleasure than pain. They eventually began removing "unnecessary" parts on her body, leaving only her erogenous zones. As their dissections continued, they began applying direct nerve stimulus, creating a far larger response than ever before. In the end, only her brain and spinal cord remained.

During the Yokohama base attack, Sumika realizes that during her ODL purification cycles with the Yokohama reactor, she has been unknowingly transmitting to the BETA details about sensitive information concerning humanity's war effort. She tells the base CO and Yuuko that the reactor must be eliminated immediately. However, the reactor is also extremely important for research against the BETA and Sumika's well being, so Yuuko attempts to get it shut down instead. Once Haruka fails to shut down the reactor, Sumika projects an image to Mitsuki that reveals the reactor's weakest points in order to thoroughly destroy it with S11 charges.

In Operation Cherry Blossom against the Original Hive, Sumika acts as the central computing unit for the Susano'o Type 4 with Takeru controlling weapons and Kasumi acting as logistics and transmission support for Sumika. Sumika shows amazingly proficient control with the mech's Rutherford field and is able to redirect an S11 explosion against thousands of approaching BETA. However, once the group manages to reach the hive's reactor, also known as the Primary Objective, they are attacked and disabled. During the attack, Sumika completely shuts down after memories of her rape resurface. Once Takeru manages to awaken her, she channels all of her power into restoring the Type 4's energy, resulting in a far stronger Rutherford field than ever before and a fully charged electromagnetic cannon.

When Takeru arrives back at the Yokohama base, he finds that Sumika died soon after he fired the final shot against the Primary Objective. Kasumi reveals to him that Sumika's subconscious jealously is what made him into a causality conductor. His continuous loops across time were the result of him engaging in relationships with other girls in Unlimited.