Iyami first appeared in the manga as a guest antagonist for the Matsuno sextuplets, initially lacking his signature traits during a brief cameo fight. His first named appearance quickly followed, establishing him as a nuisance and work acquaintance of the Matsuno patriarch. His role expanded significantly, eventually displacing the sextuplets as the protagonist in later manga runs, particularly in the Shonen King serialization focused on his bizarre situations. This shift solidified in the 1988 anime adaptation, where he and Chibita became the main focus, the opening theme centered on him, and the sextuplets moved to supporting roles.

Physically, Iyami sports three large protruding front teeth, a thin mustache, monolid eyes, and bobbed hair that ruffles when agitated. Standing approximately 170 cm, he typically wears a purple or pink suit patterned with a number ten design, a red bow tie, and patched or loose socks he insists are irreplaceable French-made. Early depictions showed him tall and slender, while later renditions, especially in the latter part of the 1988 anime, portrayed a rounder face, shorter stature, and wider limbs. His signature pose involves flexing arms and legs while exclaiming "Sheeh!" (シェー), often causing his loose sock to spring outward.

His personality blends childishness, grandiosity, and opportunism. He frequently engages in scams, theft, and lies for money or food, often justifying them as necessary to return to France. Despite being self-centered and cowardly—shifting blame and avoiding accountability—he shows occasional kindness, especially toward Chibita, like secretly providing for him while caring for an abandoned baby or helping with a lost child. He harbors strong Francophilia, insisting he lived in or "just returned" from France, despite contradictory evidence. He sprinkles his speech with French phrases like "Bonjour!" and English words like "thank you," though his actual French knowledge appears superficial. His vanity includes daily salon visits to maintain his hair.

Iyami's speech is distinctive. He uses "me" (ミー) for himself and addresses others as "chimi" (チミ) or "chimi-tachi" (チミタチ), attributed to his overbite affecting pronunciation. He ends sentences with "~zansu" (〜ザンス), a contraction of "de gozaimasu" historically linked to upper-class or theatrical speech. His laugh is written as "uhyohyohyo" (ウヒョヒョヒョ) or "uhyo" (ウヒョ). These quirks, inspired by vaudevillian performer Tony Tani, contribute to his affectedly sophisticated demeanor.

His relationships are largely antagonistic. He is a recurring rival to the sextuplets, particularly Osomatsu, whom he once tried to kill to extend his life. He shares a complex dynamic with Chibita, alternating between mentorship, partnership in schemes against the sextuplets, exploitation, and rivalry. Instances exist where Chibita dominates, acting as Iyami’s landlord. Around women, Iyami displays inappropriate behavior ranging from hopeless romanticism to lewd conduct. Inconsistent portrayals include fleeting attractions to Totoko, sometimes depicted as his fiancée in alternate settings, and familial ties like an unnamed older brother, a mother referenced in a locket, and a daughter sharing his overbite.

In spin-offs and later media, Iyami appeared as "The Director" in *Mujirushi: The Sign of Dreams*, manipulating a desperate family into an art heist. The 1993 "Osomatsu-kun Grows Up" advertisement depicted an older Iyami as a bartender in Japan, having trained in the United States, with graying hair and dentures replacing his overbite. In *Osomatsu-san: The Movie*, a meta portrayal showed him as nearly invisible in a "world of memories," symbolized by floating features like his hat and teeth, reflecting his fear of being forgotten.