Washu Fitzgerald Kobayashi is an American child prodigy who graduated from MIT at age eleven. She relocates to Japan to teach science at Umi no Hoshi Elementary School, where Sasami Kawai and Misao Amano are students. Washu moves next door to Sasami specifically to research the magical girl Pretty Sammy, aiming to scientifically analyze her magical abilities. She becomes the first human to uncover both Sasami's identity as Pretty Sammy and Misao Amano's transformation into the antagonistic Pixy Misa.

Frequently providing technological support and inventions, Washu assists Pretty Sammy, often collaborating with Sasami's father, Ginji Kawai. During these joint endeavors, they use the pseudonyms "Catherine" for Washu and "Johnny" for Ginji. One of her key inventions is the NT system. This technology supplies magical energy to Pretty Sammy, Pixy Misa, Ryo-Ohki, and Rumiya during Romio's assault on Earth, while also shielding them from Romio's own NT system and neutralizing its effects. Washu's scientific methods sometimes involve unconventional experiments, like administering an experimental "growth accelerator" to Sasami that inadvertently transforms her into a baby. Her eccentric personality includes traits such as carrying a guitar and spontaneously exclaiming English phrases like "Jesus!" when surprised or frustrated.

In the separate, alternate continuity of *Sasami: Magical Girls Club*, unrelated to *Magical Project S*, the character appears as Washu Kozuka. Here, she serves as a cooking teacher and mentor to a magical girls' club at Sasami Iwakura's school, despite her own inability to cook. She oversees the club's training and fundamentals, utilizing Ryo-Ohki in cooking demonstrations.