Eiko Arima (有馬 詠子 Arima Eiko) is Soichiro's aunt, and Shouji's elder sister. As the eldest child of the Arima family, Eiko wanted to follow her family's profession and become a doctor, but her father told her girls should not become doctors. This turned her cold and distant. She was incensed when she learned her father had a mistress and her half-brother Reiji was born. Her father's doting on Reiji left her bitter and caused her to hate both Reiji, and later his son Soichiro. Despite this, she approves of Yukino after learning of her plan to study medicine after her marriage, she merely nodded her approval and reached for a cup of tea. Shouji recognized that Eiko appreciates Yukino's ambition as similar to her own when she was young.