Sachiko Nagai is a Japanese actress and voice actress born on April 8, 1975, in Kagoshima Prefecture. She is affiliated with the theatre company Bulldogging Headrock, where she was an original member and also contributes as a海报 designer. Nagai graduated from an art department and initially aspired to become an art teacher or manga artist.

Her career began in theatre, performing in numerous stage plays with the Bulldogging Headrock troupe. Her entry into voice acting occurred by chance when a casting director for the anime The Prince of Tennis attended one of her stage performances and recommended her for a role. This led to her first and most defining voice role as Seiichi Yukimura in the 2001 anime The Prince of Tennis. She has since reprised this character consistently across the franchise's many continuations, including The Prince of Tennis OVA: National Tournament Edition, The Prince of Tennis II, The Prince of Tennis II: U-17 World Cup, and various theatrical films and games.

Beyond her signature role, Nagai's voice acting credits include Miyako Miyamori in the 2009 anime series Umi Monogatari ~Anata ga Ite Kureta Koto~, a commander in Yu-Gi-Oh! GX, and Ishida Hajime in an episode of Kakuto Kabuto Borg VxV. She also voiced a role in the video game Keroro RPG: Kishi to Musha to Densetsu no Kaizoku.

Nagai has a notable and long-standing professional relationship with actor and director Kohei Kiyasu, who is a former university classmate. Kiyasu wrote and directed many of the Bulldogging Headrock stage plays in which she performed throughout the 2000s.

A significant achievement in her career is her work on character songs for Seiichi Yukimura. Beyond performing, she wrote the lyrics for Yukimura's song Yume no Tsuzuki, a piece highly praised by the original manga artist, Takeshi Konomi, for its deep understanding of the character. Her performance as Yukimura, a role she has played for over two decades, remains her most prominent and enduring contribution to the anime industry.