Oumi's name, 淡海, means "fresh-water lake" in Japanese.
Oumi's soul symbol translates to "Trust".
Oumi is Princess Sakura's most closest lady in waiting. She trusts Oumi enough to keep hold of her soul symbol. Even when Oumi betrayed her and gave Princess Sakura's soul symbol to Aoba, Princess Sakura still felt the need to want to protect her. Oumi trusts in Princess Sakura so much that she tells her about how her family died which was because a youko killed them. She then gives up her life just to tell her that the counselor was the one behind the attacks. Oumi then turns into a Youko, but Princess Sakura couldn't bring herself to kill her hoping there was another way to save her.