Koyo Ozaki.

Koyo Ozaki is a member of the Port Mafia Executive Committee.
Chuya Nakahara and Osamu Dazai called her "Ane-san", which literally means "Older Sister".
Has the ability "Golden Demon".

Kouyou is a tall, slender young woman with cherry-red eyes and light-red hair. Her bangs, cut straight across, completely cover her left eye, while the right side of her face is completely bare. Kouyou has thin lips, which lend a softness to her smile, and slightly hooded eyes. Her hair is pulled back into a traditional Japanese bun, held in place by three long, golden pins and another hair clip on the right side of her head. The hair clip itself consists of a smaller, light-pink fluffy hair clip, a red ribbon, and ornaments that resemble "purple eyes." Together, they form a five-petaled flower, which serves as the hair clip on the right side.

Koyo wears a straight-cut black dress with a red floral pattern running along the entire hem. The outer collar (eri) of her dress is two-toned—the left side is black and the right is pink—to match the front (maimigoro) of the kimono. The inner collar (uraeri) is white. The sleeves above the elbow (furi), as well as the top of the kimono itself, are white, gradually fading to pink and red closer to the bottom. The hem is decorated with a floral pattern of pink bells. Around her waist is a wide, dark cherry-colored sash with a diamond-shaped pattern in a lighter shade. Above the sash is a burgundy bow with two white stripes at the edges. In the center of the white edging is a thin stripe, also burgundy.

Her makeup consists of red eyeshadow and light pink lipstick. Kouyou carries a red, traditional Japanese umbrella, which she uses to conceal her sword. She always carries herself with regal grace. Ozaki Kouyou, Port Mafia
Koyo wears a straight-cut black dress with a red floral pattern running along the entire hem. The outer collar (eri) of her dress is two-toned—the left side is black and the right is pink—to match the front (maimigoro) of the kimono. The inner collar (uraeri) is white. The sleeves above the elbow (furi), as well as the top of the kimono itself, are white, gradually fading to pink and red closer to the bottom. The hem is decorated with a floral pattern of pink bells. Around her waist is a wide, dark cherry-colored sash with a diamond-shaped pattern in a lighter shade. Above the sash is a burgundy bow with two white stripes at the edges. In the center of the white edging is a thin stripe, also burgundy.

Her makeup consists of red eyeshadow and light pink lipstick. Koyo carries a red, traditional Japanese umbrella, which she uses to conceal her sword. She always carries herself with regal grace.

Kouyou carries herself with grace and dignity, but wastes no effort in concealing her dark nature. Due to her failed escape from the Port Mafia, Kouyou is an extreme cynic, tormented by such things as hope and love. Ozaki disbelieves in the benefits of "light" and adheres to the ideology that those born in "darkness" are destined not to blossom for the light. In her eloquent speech, Kouyou often compares people to flowers. In her own words, "they [flowers] will never have the chance to fall if they remain mere buds." This implies that no one will remain "pure" forever and that this purity is a waste compared to time. As a result, Kouyou hates and fears "light," believing that exposure only brings disappointment and despair.

With her comrades, Kouyou maintains her composure and has a much more casual approach to interactions. She enjoys spending time with the Port Mafia Boss (Mori Ogai, always addressing him by his first name) and her former student (Chūya Nakahara).

For all her inherent grace, Kouyou's intense intelligence makes her prone to vicious attacks. Nothing unusual for a member of the Port Mafia. Ruthless and cruel, Kouyou rarely holds back in battle. The only exception is Grace An Lim. She's also not afraid to attack from behind or when her target isn't looking.

Golden Demon is an ability of Koyo Ozaki that relies on the ability to control a "phantom" at will.

Chuya Nakahara is something of a disciple of Koyo Ozaki. Chuya usually calls her Ane-san, which literally means "Older Sister." At first, Koyo wasn't too keen on this.

The name Koyo means "Autumn Leaves".
The surname Ozaki means "tail" and "cape, peninsula".