Sōsō Mōtoku, also known as Karin, rules the Gi kingdom. She first appears hostile towards Kazuto in the anti-Toutaku alliance, disdaining his appearance. She actively pursues recruiting the warrior Kan'u. During conflicts with the Hongou faction, Sōsō succumbs to Ukitsu's brainwashing. Her subordinates Kakouton and Kakouen recognize the change and seek Kazuto's aid to free her. Rescued, she surrenders and dedicates her captivity to study. Sōsō challenges Kazuto's refusal to execute prisoners, questioning his survival despite his perceived naïveté and unconventional methods. Yet she acknowledges his kindness as strength and voluntarily relinquishes rulership of Gi to him, trusting the kingdom's future under his care. Over time, affection for Kazuto grows; she recognizes his underlying insight and intelligence, eventually including him in her teasing of subordinates. She later dispatches troops to support Kazuto in the final battle against Ukitsu and Saji.

In *Shin Koihime†Musou*'s Gi route, Sōsō encounters Kazuto while pursuing bandits holding a sought book. Noticing a shooting star beforehand, she locates him in the wilderness for questioning. He references the Gi faction and addresses her by her full name despite only hearing "Sousou." At this point, China lacks established factions, and "Gi" exists solely as Sōsō's recent, unshared concept. Kazuto's apparent future knowledge convinces her of his origins, leading her to enlist him for strategic advantage in her conquests.

Sōsō's personality varies between media. The original visual novel depicts extreme arrogance, misandry, and viewing subordinates as tools, contrasting Kazuto's emphasis on camaraderie. However, she deeply cares for her people's welfare, ensuring Kazuto vows not to neglect them before transferring power. The sequel removes her hatred of men and shows quicker affection for Kazuto. She identifies his potential, placing him in challenging situations to foster growth, intervening only to correct mistakes. Though proud and ambitious, she demonstrates humility when necessary, such as apologizing to Kyocho for her village's suffering. While typically strict, she displays kindness and concern for subordinates, reacting with visible distress to Kakouton's battle injury. Her charisma naturally attracts loyal service.

The anime adaptation moderates her harshness, emphasizing honorable traits like shielding Batou from a horse accident to preserve his reputation at her expense. The manga adaptation amplifies her cruelty and superiority complex, depicting sadistic tendencies and violent outbursts when thwarted.

Sōsō wields a scythe with high skill, demonstrated by precise control like severing a single strand of Jun'iku's hair during a test. Rarely engaged in direct combat, her physical prowess exceeds ordinary soldiers, briefly impressing Kan'u. She blocks an attack from Ryofu but acknowledges an inability to withstand repeated assaults. Beyond martial skill, she excels in diverse fields: cooking (learned through observation), liquor distillation (constructed a distillery from textual knowledge), zither performance, board games, songwriting (anime-exclusive), and costume design (anime-exclusive).

Her relationships are complex. She harbors a persistent obsession with Kan'u, notably depicted in the first anime season's credits where she "kidnaps" Kan'u, later returning her covered in lip marks. In same-sex dynamics, she adopts a dominant role, employing bondage and submission, particularly with strategist Jun'iku. With the Kakou sisters, she engages in conventional lesbian intimacy. Kazuto is implied as her first male partner; after losing rulership, she becomes more submissive to his teasing. In *Shin Koihime†Musou*, her romantic dynamic with Kazuto intensifies, though she struggles to express feelings, often blaming him for misunderstandings.

In the *Kakumei* remakes, Sōsō fulfills her ambition of unifying China. A concluding scene shows her gazing at the sky, telling Kazuto she upheld her promise to become a wise and benevolent ruler.