When Kouan, the Yato's original home world, was attacked by the League of Planets, many of the planet's species died and some of the surviving Yatos fled and scattered across the universe. It is then revealed that centuries later, the Yatos that chose to stay had adapted to the mostly deserted planet. Kouka was a descendent and sole survivor of the clan and spent most of her life living on the planet alone. She would later leave the planet after meeting Umibouzu, settling down on Rakuyou to start a family with him
It is revealed in the later part of the series that Kouka is a mutant born under the effects of the Altana. Similar to Utsuro, she is immortal; she will not die as long as she continues to absorb the Altana by staying on Kouan. However, since she eventually chose to leave Kouan with Umibouzu, her health deteriorated especially after the birth of Kagura. Her condition worsened significantly after witnessing the fight between Kamui and Umibouzu, leading to her eventual death.
In the years after Kagura's birth, Umibouzu left his family's side constantly in search of a cure for her illness - Altana crystals. However, Kouka didn't want to use the crystal, preferring to spend her last days with her family.
Kouka is described as pretty by her daughter Kagura, and Umibouzu was mesmerized by her when they first met on Kouan. Like her two children, Kouka has vermilion hair. She typically wears a cheongsam, and her hair is tied in a braid with an ornament similar to Kagura's covering the top of it. She also has a single lock of hair sticking out from the top of her head, a trait she shares with her son Kamui.
Kouka appears cold at first, acting rather hostile to Umibouzu when they first met, ignoring Umibouzu's pursuit and attacking him when he sexually harassed her without looking in his direction. She also seems to be expressionless, possibly due to years of living in solitude.
She also has a terrifying side and Umibouzu fears her greatly. Mutsu describes their domestic situation as "the strongest man in the universe becomes the weakest at home." Umibouzu and Kamui are shown fleeing from Kouka in a flashback. However Kouka also loves her family very much, even willing to risk her life to stay by their side despite knowing that it will eventually cause her death.
Kouka was often mentioned by Kagura throughout the series. Although they were mostly for gag purposes, it reflected on how much her words meant to her daughter and influenced her thinking. As she passed away before the start of the series, her appearance was mostly in a form of flashback.
A bedridden Kouka briefly appeared in a flashback during the confrontation between Kamui and Kagura, where Kamui was seen hiding his injuries from her.
Kouka appeared when Umibouzu was in the midst of fighting Orochi, defeating the creature with ease. Before she left the lovestruck man, Kouka corrected his thinking by stating how Kouan was still thriving and that he should not measure the planet by his own standards. She then told Umibouzu to leave and not disturb Orochi that went aggressive due to his intrusion. She then attempt to quieten the creature down, which went on for three days and nights with Umibouzu by her side throughout. After exchanging words, Kouka introduced herself and welcomed the man back at any time as he wished. From then on they started having frequent interactions, and Kouka started developing feelings for Umibouzu. She eventually decided to follow him and left Kouan to start a family together.
Kouka led a normal life taking care of her family after settling down on Rakuyou. However her health deteriorated after giving birth to Kagura and she became bedridden since. It was also revealed that the Yato family was ostracised by the Rakuyou residents as a result of her failing health. Despite knowing she would die, Kouka did not regret her decision to leave Kouan and expressed her wishes to be with her family till the end. Kouka's condition worsened after witnessing the confrontation between her husband and son, with the latter leaving home as a result of losing.
Kouka later appeared again as Kamui's illusion while he lie on Kagura's lap after his defeat.
- Umibouzu: Her husband. They met when he came onto Kouan as a punishment for losing a bet. Though she was hostile to him at first, she was moved by his love for her during their courtship. She would later marry him and have two children together.
- Kamui: Her son. Kamui used to hide his injuries so as to not let Kouka worry about him while she was bedridden. She cares for her son deeply and felt apologetic to him for having him to shoulder the responsibilities (and damage control from the whole incident) she couldn't fulfil due to her health at such a young age.
- Kagura: Her daughter. She stayed and took care of Kouka until her death after Kamui's departure. Kagura is very attached to her and would sometimes mention her throughout the series.
- (To Umibouzu) " I told you, didn't I? I'm part of a clan that dies with the planet. I've always lived alone. I need to live alone."
- (To a young Kamui) "I'm sorry. I made you carry such heavy burdens on such a small back. But please, don't force yourself anymore. It's okay for you to live the way you want. I've been prepared for this ever since I left that planet. But the reason I stayed with your father anyhow was because I wanted to be by his side. And because I wanted to be by the side of a family I would give birth to."
- (To a young Kagura) "Thank you for always staying with me. With you around, I'm sure our family... Will be okay. "
- Kouka shares the same Japanese Voice Actress as her daughter Kagura.
- Kagura states that their father, Umibouzu, had a "shotgun marriage" and stated that "love starts in bed", so it might be possible that Kamui was born before Kouka and Umibouzu married.
- The "ko" in "江" means "bay" and "ka" in "華" means "flower" her name means "bay flower". Her name referencing how a flower that is picked will wither also referencing how when she left the planet and became ill and her subsequent death.