Kōyō is a tall and slender woman with cherry red eyes and red hair tied into a traditional Japanese bun that is held by three long golden clips and another hair clip on the right side of her head, composed by a light pink furry hair clip and red ribbon which forms a five-petal flower, and violet eyes.
Her kimono is pink overlapped by dark purple with bellflower motifs, tied neatly with a maroon bow which has white borders, and a light pink mantle which goes darker in color from her elbow to her feet. She wears a red eye shadow and light pink lipstick.
Kōyō carries a red Japanese umbrella that conceals her sword.
Kōyō holds herself with grace and dignity but wastes no efforts hiding her dark nature. As a result of her failed escape from the Port Mafia, Kōyō is an extreme cynic, jaded against things like hope and love. Kōyō doesn't believe in the benefits of 'light' and claims that people born in 'darkness' are destined to never branch out for the light. With her eloquent speech, Kōyo often equates people to flowers. Some born in darkness, some destined to wilt. In her own words, "they [flowers] never have the chance to drop... if they remain mere buds", indicating she believes that no one will always remain pure and that that purity is a waste compared to how they are when they fall. As a result, Kōyo both detests and fears 'light', thinking there's nothing but disappointment and despair once exposed.
Around her comrades, Kōyō maintains her composure but has a much more laid back approach, and enjoys leisure time with her peers, such as the mafia's boss and her fellow executive. Towards others like Ace, however, she isn't the least bit subtle in her distrust and comes off very passive-aggressive.
For all her grace, Kōyō's intense mindset leaves her prone to lashing out with cruel words and violent attacks - nothing out of the ordinary for a mafioso. Ruthless and brutal, Kōyō seldom holds back in a battle, only doing so for Kyōka Izumi's sake, and she is not above attacking when her target isn't looking.
Kōyō mentored Kyōka, consequentially being the cause of Kyōka's extreme fear of change when she first joins the Armed Detective Agency. For some time, Kōyō drilled a dreary, hopeless mindset into Kyōka to ensure she would never leave the mafia. She goes as far as to lie by telling Kyōka she's directly responsible for her own parents' murders, poisoning Kyōka's mind.
Claiming this to be only for Kyōka's well-being, Kōyō projects her frustrations with life onto Kyōka. It takes Dazai's input for Kōyō to finally stop obsessing over Kyōka's defection and let the girl grow and think for herself. Nonetheless, Kōyō's attachment to Kyōka drives her to intercept a Special Division for Unusual Powers armed transport vehicle – leaking the true events of Kyōka's past to the girl – and later makes a point of never physically harming Kyōka during the Cannibalism operation. This careful mindset extends somewhat to other detectives, however, Kōyō only states that she dislikes the thought of harming Kyōka's friends.
Far from fearless, even Kōyō panics at the possibility of suffering through Dazai's infamous torture methods.
Kōyō has the ability to control the Golden Demon (金色夜叉, Konjiki Yasha?), a sword-wielding phantom. It is the same type of ability as Kyōka's, but she can control it with her own will.
When young, she tried to leave the Mafia together with a man whom she admired. Nevertheless, they were caught and the man was killed. Since then, she has held a grudge only against the Mafia's former leader who did those heinous acts.
A year after Ōgai Mori takes over the mafia, Kōyō is assigned to oversee (not necessarily mentor) Chūya Nakahara, after he joins the Port Mafia. As a result, Chūya answered directly to her much of the time. Although she feels exasperated and too young to be called 'ane-san' (姐さん,? literally "elder sister") by the boy, it's clear she later grows to accept it when she gets older.
- Kōyō is one of the only characters to refer to Mori by his first name.
- The real-life Kōyō Ozaki was male, and his name was Tokutaro Ozaki (尾崎 徳太郎).
- In reality, Ozaki was the mentor of Kyōka Izumi, with the latter viewing Kōyō as a teacher even outside of literature.
- She shares her birthday with Yukichi Fukuzawa, on January 10.