Risako Nagisa

Risako Nagisa (凪紗 りさこ) is a teacher of the school Aika attends who first appears in AIKa R-16: Virgin Mission.

Appearance
In AIKa R-16: Virgin Mission, Risako has multiple outfits. While at school, she dresses relatively conservative while outside she prefers exclusively revealing and skimpy clothes, in a manner similar to Neena Hagen. In fact, we only see her wearing bikinis of two different types and then a full set of lingerie ever since the trip with the Treasure Hunting Club begins. In AIKa Zero, Risako wears mostly a lab coat due to her undercover role as a school nurse.

Risako is a very attractive and voluptuous women, with wide hips and particularly enormous breasts, as well as a very dangerous foe, with fighting capabilities that can even match Aika. Using her astounding looks and sensuality as a standard way of getting what she needs, she has all the characteristics of a femme fatale.

AIKa R-16:Virgin Mission
Risako is also the adviser of Treasure Hunting Club. She turns out to be an enemy, having controlled three of the students with electronic devices to attack Aika Sumeragi and Eri Shinkai and also tried to take Karen away. Her true identity is an agent from a mysterious organization sent to observe Karen.

AIKa Zero
Risako returns as a teacher to help Aika Sumeragi and watch over Karen Minamino.

Defeat
With help from Karen Minamino, Aika Sumeragi manages to defeat Risako in episode 2, Peach Peach Beach, still having quite some troubles and finding her a very difficult foe. Once she is beaten, her hands and feet are hastily tied to keep her under control. Still defiant and apparently unmoved by her defeat, Risako refuses to give any information, warns them to stop in their quest and tells them they are being overheard and reinforcements for her will soon arrive. Her arrogant attitude is cut short, however, when Karen unexpectedly pulls a gun on her and unceremoniously shoots her with a tranquilizer dart from short range. Risako then lets out a loud moan, her face blushing furiously and quickly changing to a never seen before expression of anguish and shock, as the tranquilizer dart penetrates her voluptuous body and she unceremoniously crumbles to the ground out cold.

Claiming her actions were revenge for the events of the precious day, and indifferent to the fact she just put down her teacher, Karen then approaches the unconscious body of Risako and goes even further in her humiliation of the grown up woman, partially removing her skimpy bathing suit in front of everyone to reveal a set of transmitters, the teacher's trump card. Without any chances of now receiving any help, and thus being no threat, Risako's body is carried by Gusto Turbulence to the ship, where she is left with the other captives in the deposit, restrained by several belts in such a way she can hardly even move. We can assume she remains like this for the days it takes for the mission to be over, probably taken care of, in her practically immobile state, by the ship's crew.

Power
Risako has combat skill comparable to a 16-year-old Aika Sumeragi and completely dominated her until Karen Minamino's interference. Risako was clearly stronger than Aika at this point, beating her down with no effort, straddled her and was about to finish her off until aika got the upper hand, overwhelmed and knocked Risako out because of a hot crab. Still Aika only won via interference. Before Risako was humiliating her with ease so the gap between them was that big. Risako would have easily won if not for Karen getting in the way.

In Aika Zero, Aika seems to have completely surpassed Risako as Miyu was able to instantly take her out with overwhelming speed while Aika seemed to have little trouble with Miyu. This is further shown when Risako was seen on the ground and forced to use a gun for the time in the series against Misumi while Aika was able to take down multiple White Knights with relative ease. Although the battle between Misumi and Risako isn't shown, given her position on the ground and the fact that she sighed after resorting to using firearms in battle for the first time in the series, it's likely that Risako was outmatched in a battle of pure hand-to-hand combat.

Trivia

 * The name Risako means "lovely" (麗), "village" (里), "white jasmine" (莉), "pear" (梨) or "reason, logic" (理) (ri), "help" (佐), "sand" (沙), "thread, silk" (紗) or "already, now" (早) (sa) and "child" (子) (ko).
 * Risako's surname Nagisa means "beach, shore" (渚).
 * Risako has the largest breast in the R-16 and Zero series
 * She has the most OTS carries with 4, 1 in R16 and 3 in Zero

Risako Nagisa