Which person designed Pikachu?

Atsuko Nishida designed Pikachu.

Nishida was working at Game Freak when she created the character that became Pokémon's mascot. Her initial concept drew on a squirrel rather than the mouse-like appearance many fans may assume from Pikachu's final classification and design.

The character developed during a period when Pokémon's artists wanted a cute creature to balance designs that were often more intimidating. Nishida's design included the cheek pouches and compact body that became central to Pikachu's visual identity.

Ken Sugimori is often associated with Pokémon artwork because he was the franchise's primary character designer and art director, and he finalized much of the original roster. However, the specific credit for designing Pikachu belongs to Nishida. Pikachu first appeared in the 1996 Game Boy games and became even more prominent through the anime, where it partnered with Ash.