Iyami is a recurring character who began as a minor antagonist before evolving into a central figure. His name comes from the Japanese word "嫌味" (iyami), meaning "disagreeable" or "unpleasant," reflecting his personality traits.

He possesses a tall, slender frame, large protruding front teeth frequently cited as 8.5 centimeters long, a thin mustache, and a bobbed hairstyle with a straight fringe. His typical attire includes a purple or pink suit featuring a patterned design resembling the number ten, a red bow-tie (sometimes omitted), and loose patched socks. He occasionally adds accessories like a small hat or cane. His signature "sheeh!" pose involves raising one arm, crossing the other over his chest, and springing one leg upward, occasionally resulting in his shoe flying off.

Iyami consistently asserts French heritage, declaring he "just returned from France" and peppering his speech with French phrases like "bonjour." Evidence often contradicts this claim, suggesting it is a facade motivated by Francophilia. His distinctive speech patterns include referring to himself as "me" (ミー), addressing others as "chimi" (チミ), ending sentences with "zansu" (〜ザンス), and employing laughs like "uhyo" (ウヒョ). These quirks originate from Japanese vaudevillian performer Tony Tani, who influenced his design.

His personality is childish, self-centered, and opportunistic. He engages in scams, theft, and deceit, frequently motivated by a desire to fund a return to France or survive poverty, though his plans often fail. He demonstrates cowardice, shifting blame to others, and exhibits poor hygiene, such as eating from trash cans. Despite this, he occasionally shows kindness, like secretly providing a futon and food to Chibita caring for an abandoned baby. He experiences intense emotions, from joyful dancing to grief-stricken crying, and harbors vanity about his appearance and teeth.

His relationships are primarily antagonistic. He views the Matsuno sextuplets, especially Osomatsu, as rivals and frequently targets them with pranks or schemes. His dynamic with Chibita fluctuates between mentorship, collaboration in cons, and outright manipulation; they sometimes share genuine camaraderie, while elsewhere Iyami exploits him. He displays inconsistent attraction to Totoko, ranging from inappropriate infatuation to professional partnerships or engagements in alternate scenarios. Family members include a mother depicted with red curly hair, a daughter who inherits his overbite and hairstyle, a wife, and an unnamed older brother resembling him but possessing five teeth and a sharper nose.

Iyami debuted in the manga as a background character in 1963, receiving his name in a later chapter where he harassed the Matsuno family. He evolved from a nuisance to a primary character, eventually overshadowing the sextuplets in narrative focus, particularly in the 1988 anime where he and Chibita became protagonists. In "Osomatsu-san: The Movie," a metaphorical depiction portrays him as nearly invisible—represented by floating teeth and a hat—symbolizing his fear of being forgotten. Alternate futures, such as a 1993 Sapporo beer advertisement, show him as a bartender in his own establishment after training in America, featuring gray-streaked hair and dentures replacing his overbite.