Mendo Shutaro is a major character and a prominent student at Tomobiki High School. He is introduced as the heir to the immensely powerful Mendo Zaibatsu, a conglomerate so vast that his family is said to own a majority stake in Japan and possesses its own private army. Born on April 1, he transfers to the school, immediately commanding attention with his handsome features, athletic prowess, and an almost theatrical air of sophistication, often arriving by parachuting from his private jet.

Despite his impeccable grooming and noble bearing, Mendo’s personality is a contradictory blend of charm and pettiness. He presents himself as a gentleman, speaking politely to women and displaying an outward respect for authority figures like his teacher, Onsen-Mark. However, this veneer quickly cracks, revealing a core that is remarkably similar to his rival, Ataru Moroboshi. Mendo is deeply lecherous, professing romantic interest in nearly every girl he meets while treating his male classmates with open disdain. He is extremely vain and easily insulted, with his pride being his most fragile asset.

His primary motivation is a persistent and unrequited love for the alien Lum, whom he respectfully calls Lum-san. He views Ataru, Lum’s true love interest, as his greatest enemy and goes to great lengths to humiliate or physically harm him, often by ordering his Black Glasses security squad to open fire or by attempting to strike him down with his ancestral katana, Murasame. Despite this bitter rivalry, the two share a strange, unspoken bond; they are frequently seen spending time together, and their constant bickering masks a bizarre friendship, though neither would ever admit it.

In the story, Mendo serves as a foil to Ataru and a catalyst for many chaotic situations. He is a direct rival for the attention of the female characters, including Shinobu Miyake, who ends her relationship with Ataru to pursue Mendo, though he never reciprocates her feelings with any particular favoritism. His immense wealth and resources allow the series to explore scenarios involving exotic pets, such as his beloved pet octopus Matsuchiyo, and grandiose, often absurd, adventures.

A defining trait is his intense nyctophobia and claustrophobia, an irrational fear of darkness and enclosed spaces. When trapped in a dark, cramped area, typically inside a large temple bell thanks to Ataru’s mischief, he loses all composure and screams his famous catchphrase, "It's dark, it's cramped, I'm scared!". However, his vanity is so powerful that these phobias temporarily vanish in the presence of a woman who might witness his panic. While he once attempted to cure these fears through harsh training, they remain a permanent and exploitable weakness. In a time-travel storyline, it is revealed that the origin of these phobias is a self-inflicted paradox, where his own adult self terrorized his younger version, creating the trauma he would carry into the future.

Mendo’s relationships are complex. He is constantly trying to win Lum’s affection, but she only shows him polite indifference, occasionally zapping him when he bullies Ataru. He has a complicated dynamic with his younger sister, Ryoko, whom he loves but is also rightfully wary of due to her sadistic and manipulative nature. His family crest is the octopus, he keeps several as pets, and he is notably offended if anyone eats takoyaki in his presence. While his skills with a sword are formidable, his attempts at violence are almost always comically ineffective against the more resilient characters.