Hideyoshi Sarutobi is a classmate and friend of the protagonist Nagasumi Michishio in the anime My Bride is a Mermaid. Universally referred to by the nickname Saru, which is the Japanese word for monkey, he is also called Chimp in some adaptations. This nickname is a direct pun on his character and a running joke throughout the series, fitting his mischievous and agile demeanor. Physically, he is an adolescent boy with brown hair cut to ear length and is typically seen in his school uniform. His eyes are often drawn as closed or not readily visible, adding to his perpetually scheming expression, and he sports distinct sideburns.

Personality wise, Saru is defined by his overtly perverted tendencies and a notable lack of moral backbone. He frequently manipulates his friend Nagasumi into participating in risky and lecherous schemes, such as attempting to peep on girls. This self-serving nature extends to his sense of self-preservation; when these plans inevitably backfire, Saru has no qualms about abandoning Nagasumi to face the consequences alone while he escapes unscathed. Despite his constant failures and the severe physical punishment he often receives as a result, he displays a strange resilience. He endures comically exaggerated injuries that require frequent visits to the school nurse, yet he never suffers any lasting damage and sometimes even seems to derive a strange enjoyment from the harsh treatment.

Saru's motivations initially appear to be shallow, driven by personal pleasure and financial gain. This is most clearly demonstrated when the wealthy and charismatic Kai Mikawa transfers to their school. Recognizing an opportunity for profit, Saru quickly ingratiates himself with Kai, becoming his self-proclaimed servant. In this role, he adopts playful ninja-like mannerisms, a gag that ties back to his monkey namesake and the historical ninja Sarutobi Sasuke. His name also draws from Toyotomi Hideyoshi, a historical figure who was himself nicknamed Monkey. However, his character shows notable development in his relationship with Kai. While his loyalty is initially bought, it gradually evolves into genuine care and friendship. In a specific episode, he demonstrates authentic concern for Kai's well-being, proving his affections run deeper than simple monetary interest.

Beyond his role as Nagasumi's troublesome friend and Kai's devoted servant, Saru is also the source of a recurring comedic persona known as Saru Roshi. Appearing dressed as a lecherous old hermit, this alter ego is a clear parody of Master Roshi from Dragon Ball. In this guise, referred to as the ero-sennin or perverted sage, he offers unsolicited and perverse advice to the other characters, further cementing his role as a source of lowbrow comedic relief.