Nishi is the daughter of a teacher and was destined to become a teacher like her father. She wasn't allowed to read manga in her youth, but she made up for that lack in college. She was discovered in a June manga-contest headed by Takemiya Keiko. Still a college student, she made her debut in 1988 with "Matte iru yo" [Wait here] in Petit Flower. She did become a teacher but could not keep up combining the two and quit her teaching job after six months to become a full time manga artist.

Nishi Keiko was the illustrator for the early volumes of Fujimi Orchestra (novel version).