When Enishi was an infant, his mother died and he was raised by his older sister, Tomoe. She raised him herself, becoming both sister and mother to him, and he grew to love her quite dearly.

When Tomoe became engaged to Kiyosato Akira, Enishi reluctantly accepted him because Kiyosato truly made Tomoe happy and she loved him. Kiyosato later joined the Mimawarigumi believing that the sombre Tomoe would be happy if he became a hero, only to be killed by Hitokiri Battōsai (Himura Kenshin) for the new era. Tomoe leaves her brother behind to seek revenge, working with spies from the Shogunate to do so. Enishi becomes consumed by his loneliness to bring her back to Edo (now Tokyo) so they could be together.

(Source: Wikipedia)