Lion-sempai is an anthropomorphic lion occupying a middle management position in a Japanese corporate office, typically wearing a gray suit that contrasts with subordinate Lizard's olive green attire. Despite his imposing size, species-based physical intimidation, and "King of the Jungle" status, he possesses a fundamentally warmhearted, kind, and decent personality. This disconnect between his formidable appearance and genuine kindness creates significant social challenges: colleagues and smaller office animals frequently fear him, mistaking him for predatory or aggressive. He remains largely oblivious to the terror he inspires, misinterpreting his nickname "Big-cat" as respect rather than fear.

As senpai to two subordinates—Lizard, a serious and cynical pogona lizard, and Toucan, a perverted and trouble-prone bird—Lion strives to be an ideal boss and mentor. He works diligently, taking responsibilities seriously, but often manages chaos from Toucan's reckless, womanizing, or lazy behavior while navigating unreasonable demands from superior President Turtle. Lion's attempts to uphold professionalism and team harmony define his office interactions.

His personal challenges include age-related farsightedness and a specific aversion to needles, which he attributes to a fear of blood. This manifests in refusals to participate in blood drives, though his reactions to media violence or Lizard's frequently detached and bleeding tail—dismissed as Lizard's "default state"—suggest his fear centers on medical needle procedures.

Physically expressive, he humorously mimics the Peugeot logo by posing sideways and roaring. Species-based communication difficulties arise in scenarios like a dental appointment, where his roars of pain during a cavity filling were misinterpreted as aggressive threats by the dentist. This required intervention from Toucan and Lizard as sign-language translators.