Bachou Mouki, born Sui, hails from Ryoushuu as the daughter of Batou. Her father dies by suicide when Sousou Moutoku's forces invade their homeland, rendering her homeless and consumed by a desire for vengeance against Sousou. She seeks out the Shoku faction, presenting herself to Kazuto to join his army, initially fueled solely by revenge. Over time, she experiences emotional healing, gradually processing her grief and ultimately reaching a point of forgiving Sousou. Her path transforms from seeking retribution to discovering peace and purpose within Shoku.

She exhibits a tomboyish personality, marked by straightforwardness and a preference for physical action over intricate intellectual strategies, openly acknowledging her limitations in complex planning. Fierce loyalty to her allies and duties defines her. She possesses a significant complex about her femininity, reacting with intense embarrassment to romantic or suggestive subjects. Kazuto frequently exploits this trait, deliberately teasing her into flustered reactions or stuttering. These interactions often escalate to tsundere-like behavior where she defensively calls Kazuto "baka" and responds with physical hits. Outside these provocations, she remains easily embarrassed by compliments about her appeal or discussions of traditionally feminine topics.

Her combat prowess is formidable, centered on mastery of a cross-shaped yari, a weapon designed for both slashing and thrusting. This spear technique is augmented by enhanced physical abilities: exceptional strength, speed, reflexes, and stamina. She excels in cavalry command, a skill rooted in her family's equestrian heritage in Ryoushuu, ranking among her faction's best riders. She owns three named horses: Ouhou, Shien, and Kirin. She also possesses the insight to gauge an opponent's strength and honesty through their aura. Notable weaknesses include impulsiveness, a tendency to jump to conclusions, and mild urinary incontinence, occasionally used for comedic effect when she is prevented from bathroom access.

Relationships are central to her development. She forms a close friendship with Chouhi Yokutoku; they are often grouped as Shoku's less intellectually inclined members, but their bond is characterized by mutual respect and battlefield camaraderie. Her dynamic with Kazuto evolves from initial distrust to deep, albeit unspoken, affection, despite her frequent denials of romantic feelings. Accidental physical interactions with Ryūbi—like falling onto her or holding her hand—spark misunderstandings, such as Kan'u questioning if she impregnated Ryūbi, which she vehemently denies.

Additional traits include her large appetite, ranking her among the series' top four eaters alongside Chouhi, Ryofu, and Kyocho. Her family martial arts regimen involves intense training routines: bunnyhopping with weights, inverted situps, and volcanic crater hand-walking. Later visual novel iterations depict her with thicker eyebrows. Her design features a short skirt, contributing to a "gap-moe" effect. In non-anime combat situations, she wears a headband. Animated adaptations portray her as a notably heavy sleeper.