Souei hails from the Ogre tribe, originally serving as a retainer alongside Benimaru. His tribe fell to the Orc Lord, forcing him to flee to the Jura Forest with five fellow survivors. After an initial confrontation with Rimuru Tempest due to misunderstanding, Souei pledged allegiance upon learning the truth of his tribe's destruction. Rimuru named him "Souei," triggering his evolution from Ogre to Kijin. This transformation gave him the appearance of a lean young man with dark skin, blue-black hair, sapphire eyes, and a single white horn. His attire, designed for espionage, features a dark blue color scheme balancing mobility and stealth.

As Rimuru's Minister of Intelligence, Souei leads the covert unit Kurayami (Shadow Squad). Following Rimuru's ascension to Demon Lord, Souei evolved further into a Youki and then a Kishin (Dark Spirit Ogre), earning the title "Darkness." He manages surveillance, counter-espionage, and sensitive missions across Tempest and beyond, utilizing subordinates like Souka and Ritus. He maintains a private communication line to Rimuru via specialized thread.

Souei exhibits a calm, stoic demeanor with occasional chunibyo traits, taking his ninja role intensely seriously. Typically emotionless, he displays ruthless efficiency against threats to Rimuru, often smiling ominously when angered. He interprets orders pragmatically, sometimes instilling fear beyond directives to achieve objectives. In non-combat settings, he demonstrates subtle humor, such as teasing Souka about her affections or showing fondness for cats. He maintains a gentlemanly demeanor toward women but remains emotionally reserved.

His core abilities focus on espionage and assassination. He initially wielded the Unique Skill "Shinobumono" for stealth and clones, later acquiring the Ultimate Gift "Tsukuyomi." This enhanced his capabilities with Parallel Existence for multiple bodies, Spatial Manipulation for teleportation, and Universal Perception for sensory coverage. He employs "Nenkou Threads" for combat—sticky, steel-hard threads used for techniques like Mystic Thread Binding (restraint), Mystic Thread Slash (cutting), and Puppet Strings (mind control). He also wields dual ninjatō and kunai for direct engagements.

Souei shares a lifelong rivalry with Benimaru, transitioning from retainer to equal after joining Tempest. He collaborates professionally with Diablo on intelligence matters, though he finds their shared interests disconcerting. His deputy Souka harbors unreciprocated feelings for him, which he acknowledges but does not openly return. He maintains distant respect for other leaders like Shion, who humorously warned Souka about his eccentricities.