Sei Ashina was born Igarashi Aya, and assumed the stage name Sei Ashina. Prior to her acting career, Ashina was a model. She made her acting debut in the 2002 TBS Television series The Tail of Happiness (しあわせのシッポ）, modeling that year in magazines including Shogakukan's CanCam and Shueisha's Pinky, and was best known for starring as Hime in Kamen Rider Hibiki. She was also known for her role as The Girl, an unnamed concubine, in Silk (2007).