Yotsuba Koiwai

Height: 107 cm

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 "Yotsubox" 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. Yotsuba has never attended school, and as of the first chapter does not know what a grade is. In chapter 35, she fails to understand repeated explanations of homework.