Momiji is a female member of the long-nosed tribe, also known as the tengu, from a hidden village in the Kusha Mountains. She is the daughter of the elder Kaede and the renowned swordsman Hakurou. Her name, which translates to Autumn Leaves, was given by her mother after a three-century gestation that drained much of Kaede's energy. Following her birth, Momiji was entrusted to Hakurou's care with the hope that she would progress in her training, improve her skills, and form a bond with her father. She currently resides in Rimuru City, the capital of the Monster Nation.

Momiji is a determined and dutiful individual who takes her responsibilities seriously. Despite her relatively young age, she acts as a proxy for her mother, the tengu elder, and represents the tengu clan in diplomatic and military matters. Her motivations center on proving herself as a capable warrior and leader, as well as honoring her family's legacy. She is respectful and composed, but also possesses a strong will and a sense of pride in her heritage.

In the story, Momiji serves as a samurai commander within the Monster Nation's army, contributing to the defense and administration of Rimuru's domain. Her role expands when she becomes the first wife of Benimaru, one of Rimuru's most powerful subordinates and a key figure in the nation's military. This marriage solidifies the alliance between the tengu and the Monster Nation, and Momiji later becomes pregnant, continuing the lineage of the tengu elders.

Momiji's key relationships include her father Hakurou, under whose tutelage she hones her swordsmanship, and her mother Kaede, from whom she inherits leadership duties. Her bond with Benimaru develops from mutual respect into marriage, though she must navigate her position alongside Albis, who becomes Benimaru's second wife. She also interacts with Rimuru and other leaders of Tempest, establishing herself as a loyal and reliable ally.

Momiji's notable abilities stem from her tengu heritage and rigorous training. She is a proficient swordswoman, having been taught by Hakurou, one of the most skilled swordsmen in the series. She also demonstrates competence in command and strategy as a samurai commander. Her long gestation and tengu physiology likely grant her enhanced physical capabilities and longevity, though specific magical or unique skills are not prominently detailed in available sources. Through her marriage to Benimaru, she becomes closely tied to the central events of the Monster Nation's growth and the broader conflicts of the world.