Keiko Arima

is the wife of Kodai Arima and the biological mother of Takuya Arima who is dead prior to the series' beginning.

Etymology

 * The name Keiko means "favor, benefit" (恵) (kei) and "child" (子) (ko).
 * Keiko's surname Arima means "exist" (有) (ari) and "horse" (馬) (ma).