Kei Ohno is the older sister of Hikaru Ohno. Though she is 26yo. Her body stopped growing when she was younger.
Kei Ohno is a main character of the manga series Take On Me (known as Domin-8 Me in U.S.) that was created by Sessyu Takemura. It originally began publication by Core Magazine's Megastore Comics in 2004.
Her first appearance was in Take On Me Volume 1 CH. 2 "Big".