Oumi has long hair with a clip on the left side. Just like all of Hayase's descendants, she has eyes that strongly resemble Hayase's and wears the same clothes as all the previous and future leaders of The Defense Corps. She was pregnant with her daughter Ushio at the time she met Fushi.
Although Oumi isn't shown interacting with Fushi for long, it can be assumed that she was similar to her mother, being too friendly towards Fushi.
It can be assumed, as with all other descendants of Hayase, that she has been taught to love and protect Fushi.
Oumi is introduced in Chapter 61. She informs Fushi of The Defense Corps activities, explaining that they have been working on gathering more allies and are working towards a plan to counter the Knockers over the entire Hena continent. She goes on to explain that a flag is raised in the towns that agree with their activities. This flag is used to show that the citizens are under the protection of The Defense Corps and are prepared for a Knocker attack. She tells Fushi that if a Knocker appeared in one of those towns, the citizens would surely fight with him.
Fushi points out that the sign on the flag is a Janada mark, but Oumi says that is an old interpretation and that it now it is now the symbol of "those entrusted by God." Fushi asks if the Knocker on her arm ever resists; after all, it is basically betraying it's kind. Oumi says there is some discord at times, but compared to other Knockers it is very cooperative.
Fushi then asks her if she is Hisame's daughter and Omi happily says yes, she is. Fushi, hesitantly and nervously, asks Oumi who her father is. She tells Fushi to relax because there is no way he is her father, as Hisame had her when she was seventeen. She then tells Fushi that, from what she understands, he is a very kind person who sympathized with her mother in her youth.
Oumi, who is revealed to be pregnant, tells Fushi that Hisame died half a year ago and that the child in her womb will be next to succeed Hayase's will. She hopes her daughter will enjoy Fushi's care. Fushi awkwardly asks Oumi to leave.