Souei began as an Ogre tribesman with a lean, muscular build, dark skin, dark blue hair, and blue eyes, marked by a single white forehead horn, blue eye-shadow, and prominent white canines. After his tribe's destruction by the Orc Lord, he fled with Benimaru and others into the Jura Forest, encountering Rimuru Tempest. Named by Rimuru, he evolved into a Kijin, adopting a more humanoid form around 190 cm tall, retaining a lean physique, calm presence, and refined features. His attire is typically dark blue and functional, suited for espionage. Further evolutions to Youki and Kishin (Dark Spirit Ogre) maintained this core appearance. He earned the title "Yami" (Darkness).

Personality-wise, Souei exhibits stoicism, extreme composure, and minimal emotional display. He holds intense loyalty to Rimuru Tempest, executing orders without personal feelings and reacting ruthlessly to perceived threats or insults against his master, sometimes bending commands to instill fear. He possesses a noted affinity for theatricality concerning his ninja role, described as "chunibyo," taking it exceptionally seriously in peacetime without compromising duties. He speaks with calm confidence and eloquence. Interactions reveal a subtle dark sense of humor, particularly when teasing his subordinate Souka, though he generally remains detached. His demeanor is gentlemanly towards women but maintains professional distance during covert operations.

Souei's abilities center on espionage, intelligence gathering, and precision combat. Initially using ninja swords, his signature techniques involve manipulating specialized threads: Nenkou Threads (sticky, steel-hard threads for binding, cutting, or controlling enemies), Demonwire Bind, and Puppet Strings for neural manipulation. He utilizes Shadow Motion for traversing shadows and Spatial Motion for teleportation to known locations. His Replication skill creates up to six clones sharing his skills. Sensory abilities include Magic Sense for environmental awareness and Observing Eyes for detecting magicule fluctuations. Post-evolution and receiving Rimuru's gifts, he acquired the Ultimate Gift "Tsukuyomi" (Moon King), enhancing shadow manipulation, enabling Parallel Existence for numerous fully autonomous clones, and achieving instantaneous teleportation for real-time surveillance over vast areas. This evolution also granted heightened resistances, including Pain Nullification and resistance to physical, spiritual, and holy attacks. His Energy Points (EP) post-evolution are 1,281,162.

Souei serves as Rimuru's chief intelligence operative and leader of the covert Kurayami (Shadow Squad) in the Jura-Tempest Federation. He established a counter-espionage network capable of thwarting threats like Dwargon spies, though purely magical espionage sometimes requires expert assistance. He possesses a unique "personal line," a special thread from Rimuru enabling instantaneous, private communication. His relationship with Benimaru is defined by lifelong rivalry and deep comradeship, shifting towards greater equality after significant evolution. He mentors Souka, the Kurayami deputy leader who harbors strong admiration for him; he acknowledges her feelings but responds with detached amusement or mild teasing. He maintains professional respect within Tempest's leadership, occasionally sharing a darkly humorous rapport with Diablo, reportedly without encouraging this association.