Yotsuba is drawn as a small girl with green hair done in four pigtails, giving her somewhat the appearance of her namesake, a . She has a carefree and energetic personality, taking delight in simple matters even as she learns about all manner of things in her daily life. In Japanese, Yotsuba's dialogue is written without kanji, making it seem simpler and more child-like, and in a typeface that gives the impression of speaking with high intensity. Her energy is noted by other characters, especially members of the neighboring Ayase family. Her father says of her carefree nature, "She can find happiness in anything. Nothing in this world can get her down." However, when deeply frightened or upset, she does cry, and she has an unexplained fear of anything resembling a bullseye.
At the start of the series, Yotsuba is shown as having very little knowledge of the world around her, even for a young child. Things such as swings, doorbells, cicadas, and air conditioners all fascinate and confuse her, although she is not perturbed by her ignorance. She occasionally mispronounces new words and creates neologisms, such as as a portmanteau of "Yotsuba's Box", and often repeats, in incongruous ways, phrases spoken by adult characters around her. Yotsuba is frequently shown drawing, though she is not as good an artist as she thinks she is, and she is an excellent swimmer.
The series provides few details about her life before its start. She is an adopted child, with her birthplace unknown to the reader, although she claims she's from an island "to the left." Koiwai, Yotsuba's adopted father, says he met her as an orphan in a foreign country and before he knew it he was raising her as his own; she is sometimes taken for a foreigner by strangers. When asked about her mother, she doesn't understand the question, and she gets confused by the concept of having two sets of grandparents. Before moving to her current home, Yotsuba lived in the country with Koiwai and his mother. She initially claims to Fuka Ayase she is six years old, but her father later corrects this, saying she is in fact five years old.
The character of Yotsuba is cited by reviewers as one of the key appeals of Yotsuba&!, especially her energy, enthusiasm, and sense of wonder. For example, one wrote, "Yotsuba’s wide-eyed awe at each discovery, from the idea of a milkman to learning how to catch fish, is both inspiring and infectious. You want to see what happens next, because she continually comes across as genuine without turning into cloying." A reviewer at Anime News Network wrote, "What is really special about Yotsuba, though, is that newness with which she, as a child, sees the world. That the manga allows us to glimpse the world through those same eager eyes is what gives it appeal far beyond its humor." Another claimed that "Yotsuba Koiwai's adventures are ... a lucid and charming look at the world through a child's eyes, as she gets into scrapes that remind us all of our own childhoods (if only through manga-tinted glasses)."
Reviewers often describe Azuma's depiction of her as realistic, especially compared to depictions of children in other manga and anime. One review claimed that "Yotsuba in particular is amusing, because she acts and speaks with that peculiar mix of honesty, immediacy, and childish logic that only young children seem to possess ... Yotsuba isn't a silent, simpering sweetie-pie, she acts like a real four-year-old." On the other hand, Tom Spurgeon claimed Yotsuba is "an idealized kid of that early age, retaining a wide-eye wonder and furious energy, minus the things that crop up at that age like cruelty and deception" and a reviewer in Newtype USA said that "Her hijinks are sweetly innocent, like a cuter, more naїve version of Dennis the Menace minus the 'menace'."