Yotsuyu is a Doman-Midlander Hyur with fair skin, pale yellow eyes, a beauty mark above the left side of her chin, and long dark hair with an ornate flower hairpin. She wears an elegant black collar with a pattern reminiscent of a spider's web, a black robe with red accents, and a gold obi. She is rarely seen without a smoking pipe.
Following her memory loss, Yotsuyu wears peasant clothing with a grass hat.
Yotsuyu's grace and beauty belie a heart of stone and a will to survive by any means. A cruel, sadistic ruler, she scorns Domans due to her painful childhood and marriage to an abusive noble. Under the pretext of purging radical elements, and satisfying her personal arousal from seeing such inhumane acts on Domans carried out, Yotsuyu perpetrates numerous atrocities against her own people. Yotsuyu herself sees her hatred for Doma like an abyss, bottomless and insatiable.
Though loyal to the Garlean Empire, specifically Zenos yae Galvus, Yotsuyu is verbally abusive to her subordinates, Grynewaht especially, to get them to obey her. She fears the Garlean prince, for she knows he does not tolerate failure and thus is desperate to stay upon his good graces.
Yotsuyu wasn't always hateful, but rather depressed and lonely, especially during her early childhood and young adult life. Her former master, when she was a courtesan, described her when her father sold her as "more doll than woman" and having "eyes that had given up on life." Her rage and hate surfaced when she was given the chance by the Garleans.
Following her memory loss as "Tsuyu", she becomes more child-like and vulnerable, always at Gosetsu's side for safety. When confronted by the people whose lives she made miserable, she falls to her knees crying, apologizing, despite having forgotten her past transgressions. Yotsuyu's true goal was to make her family pay for the torment they made her endure, for she accepts that they made her the monster Doma fears and hates.
Born in the Naeuri family, Yotsuyu suffered a terrible childhood before her parents married her away to an abusive noble named Sashihai. She was sent to a pleasure house to settle her husband's debts after he died. She betrayed her people to serve as a Garlean spy for Zenos yae Galvus, and became acting viceroy of Doma by Zenos's decrees, counting on her hatred to remove all hope for the Domans.
Yotsuyu uses her position to torment the Domans while denouncing the Confederacy of the Ruby Sea as a legitimate governing party. She announces that those who do business with them will be branded enemies of the empire.
With the recently demoted Grynewaht as her bodyguard, Yotsuyu holds court in the fishing village of Isari to satisfy her hatred on the grounds of weeding out traitors and undesirables. Gosetsu Everfall, watching her actions with the Warrior of Light, Lyse Hext, and Alisaie Leveilleur, turns himself in to stall for time as Yotsuyu interrogates him on the whereabouts of Doma's true heir, Hien. The Scions of the Seventh Dawn win over the Confederacy and drive the imperials out of Isari once they force the Red Kojin to abandon their imperial posts. Yotsuyu later has Grynewaht slaughter Red Kojin before taking the katana Ame no Habaki, which the Beastmen had among their treasures.
Yotsuyu presents the blade to Zenos when he comes to Doma Castle upon hearing of the Confederate attack. He tests the blade on the Warrior and Yugiri Mistwalker when they attempt to assassinate him. Grabbing Yotsuyu by her hair, Zenos gives her a final chance to end the rebels before throwing her to the ground. After slapping Grynewaht when he attempts to help her up, Yotsuyu reveals the Warrior's presence on Yanxia. She orders Grynewaht to kill the Warrior of Light's group while preparing a massive operation to purge all opposition in Doma.
The Doman Liberation Front refuses to accept defeat. Yotsuyu commands her forces to fight to the death and has the cyborg monster that Grynewaht became act out his last attempt of revenge as her last line of defense. After Grynewaht is defeated, Yotsuyu is cornered by Hien and his vassals. She explains why she sided with the empire and reveals she was buying time for her final gamble: have Doma Castle collapse on itself to take the prince with her. She is cut down when she attempts to shoot Hien, and tries to kill Gosetsu when he holds up the ceiling. Yotsuyu fails to stop Hien's group from escaping as she is seemingly crushed under the sinking castle. Carried by the current, Yotsuyu and Gosetsu survive and end up stranded on an island.
In "The Legend Returns", when Gosetsu gets himself and Yotsuyu to Kugane via a makeshift raft, Yotsuyu suffers memory loss. She has become childlike, and Gosetsu refers to her as "Tsuyu". She obediently accompanies Gosetsu to find passage across the Ruby Sea to Doma to reunite with Hien, with the retainer concerned for her due to her past actions against the Doman people.
In "Under the Moonlight", Gosetsu remains bedridden and hospitalized by Tsuyu. He claims to be healthier and to be back in fighting condition soon. After wanting to taste the kaki fruit of Namai, his favorite food when he was a child, Tsuyu runs off to the village to search for it. Her memories have been gradually returning and she realizes they are actually something she did to the people of Doma. She kneels and bows down, apologizing to the villagers of Namai. Lord Hien asks the villagers to leave this matter to him and brings Tsuyu back.
During the meeting between Lord Hien and Asahi sas Brutus to confirm the negotiation on captives trading, Asahi brings out his parents who are Yotsuyu's stepparents, which has a critical effect on her amnesia. After Lord Hien's party returns to Kienkan, Tsuyu runs away and regains her memories. Thinking that she doesn't deserve Gosetsu's kindness, she tries to commit suicide. Her stepparents believe she still has no memories of the past, and discuss how they're going to sell Yotsuyu for money again. Yotsuyu murders them and returns with Asahi who admits he sent his parents to be killed by Yotsuyu.
In the conclusion of the events leading up to "Castrum Fluminis", Yotsuyu insists on playing her role as acting viceroy of Doma as Zenos ordered her, the people being hers to govern and to punish. As Hien contradicts her claim, her iconic cruel expression hushes over her face. She calls Doma a putrid, pestilent swamp which she is supposed to reign over until the very last man has been broken by her.
She takes out her moon-crested artifact and summons the essence of Tsukuyomi, divinity of the night, splitting her personality under the influence of the moon-related primal. She engages Hien, the Warrior of Light and companions, transforms her weapons (the pipe-gun and dual-wielded katanas) into more powerful forms, and combines them with the freshly-obtained powers governing the moon phases, to devastating effects. Tsukuyomi shortly dissipates only to engulf the area with what appears as Yotsuyu's battered spirit, who realizes the primal feeds on her spite and suffering. Spectres of Yotsuyu's former relatives, underlings and familiar people are summoned to the site, parading fragments of her tragic past experiences before the eyes of the Warrior of the Light and everyone present. The flood of memories and emotions sends Yotsuyu into a state mixed of masochism, self-pity and wrath.
Convinced she has nothing more to lose she gives in to the suffering, calling on the past's ghosts to fuel her soul with blackest malice. Only Gosetsu's "ghost" tries to intervene and pleads her to not throw away her life again. The Warrior of Light's party triumphs over her, releasing the primal's grasp from Yotsuyu, ending her path of vengeance.
Yotsuyu returns to her original form, barely alive, yet Asahi shoots her several times with his pistol. Asahi begrudges Zenos giving her everything he believes should be his and kicks Yotsuyu lying on the floor until she summons Tsukuyomi's blades to kill her stepbrother, saying that she saved the last of her strength just for him.
Yotsuyu claims the witch of Doma is about to die, and Gosetsu might mourn for "Tsuyu". Asahi's last words foretell Zenos coming for the Warrior of Light. Lord Hien comes back with his retainers. Gosetsu mourns for Yotsuyu, asking why the kami had to save their lives just to inflict this pain and tragedy.
Yotsuyu means "night dew" in Japanese.
After losing her memories, Gosetsu gives her a nickname "Tsuyu".Tsuyu means "morning dew" in Japanese.
There's a main scenario quest involving her titled "Gone Like the Morning Dew".