Hideyoshi Sarutobi, universally nicknamed "Saru" or "Chimp" referencing the Japanese word for monkey, is a classmate and friend of Nagasumi Michishio. He exhibits overtly perverted tendencies and a lack of backbone, frequently manipulating Nagasumi into risky situations like peeping on girls. Saru displays a self-serving nature, abandoning Nagasumi to avoid consequences when schemes backfire and aligning with individuals he perceives as advantageous.

Financially motivated, he ingratiates himself with Kai Mikawa upon the latter's school arrival, becoming Kai's self-proclaimed servant. Saru adopts ninja-like mannerisms in this role, reflecting his "monkey" namesake as a running gag. Though initially driven by monetary gain, his loyalty to Kai evolves into genuine care, highlighted in episode 24 where he demonstrates authentic concern for Kai's well-being.

Saru frequently endures physical punishment for his actions, suffering comically exaggerated injuries that necessitate frequent school nurse visits. Despite recurring beatings, he displays resilience, showing no lasting damage and occasionally seeming to derive enjoyment from the harsh treatment.

An additional recurring comedic element involves Saru adopting the "Saru Roshi" persona—a lecherous old hermit parodying Master Roshi from *Dragon Ball*. Dubbed "ero-sennin" (perverted sage), he dispenses perverted advice in this guise. His character name combines historical references to Sarutobi Sasuke (a folkloric ninja) and Toyotomi Hideyoshi (a historical figure also nicknamed "Saru/Monkey").