Washu Kozuka outwardly serves as the cooking club advisor at Sasami's school, a cover for her true role as secret mentor to the Magical Girls Club. She operates undercover as a "storyteller," a human agent dispatched by the witches' dimension to guide young magic users. Her initial mission focused on teaching the girls to suppress their powers for seamless integration into human society. This directive shifted dramatically when she uncovered a conspiracy within the witch hierarchy; she then retrained them to actively wield their magic.

Born human without innate magic, Washu discovered the witches' dimension and chose to abandon humanity, driven by shared disillusionment. Her background before teaching suggests work in healthcare, indicated by her frequent white lab coat and references to being hired as a nurse or health teacher. Unlike humans like Ginji (Sasami's father) who gained magic near the dark cauldron, Washu remains non-magical. Her allegiance to the witches later fractured when their plans threatened Earth.

Assertive, rebellious, and quick-tempered, Washu often exhibits tyrannical behavior, such as punching colleague Itoki during an argument. Despite a harsh exterior and occasional laziness in maintaining her cover, she demonstrates profound care for the magical girls, repeatedly risking her life to protect them. She collaborates closely with fellow storyteller Daimon, defying witch orders without romantic involvement. Her pet Ryo-Ohki suffers mistreatment from her, leading it to bond more closely with Sasami.

Physically, she retains the crab-like hairstyle associated with other franchise iterations but possesses distinctive red eyes instead of green. Lacking the scientific genius, inventions, or eccentric quirks of other Tenchi Muyo! versions, she relies solely on bravery and mentorship, directly training and emotionally supporting the girls without technological aid.

Her character arc centers on escalating defiance against the witches. Discovering their plan to destroy Earth under the Chief Sorceress's orders, she openly rebels despite being outmatched. This includes retraining the girls for combat and moral challenges, particularly during the second season's conflict. Her journey culminates in prioritizing the girls' safety and autonomy over her original mission, revealing her deep loyalty and protective instincts.