Yotsuba will deliver Fuuka her milk, even if she hasn't a clue where Fuuka's school is, or even what a school is. Because a milkman delivers.
Also, when told to return home alive from "exacting revenge" she replied: "Even if I die I'll come home alive." And she did.That's determination.
Expressive Hair: Her pigtails rise and fall with her (rapidly changing) moods.
Genki Girl: In spades. And the trump suit. This gets Lampshaded by Asagi: "Her default rate of movement really is running."
Happily Adopted: She doesn't care one bit about her parentage. Koiwai is "dad" and that's that.
Hate at First Sight: Yanda tried to enter the house while her dad was away. Yotsuba was taught not to let any strangers in when she's alone. Which leads to her slamming the door on his face. The fact that Yanda enjoys making her angry on purpose doesn't help improve her opinion of him.
Malaproper: She mostly knows the words, but how to say them is still a little beyond her.
Meaningful Name: It means "four leaves" as in "four-leaf clover". Thus her hair.
"Yotsuba" and "Koiwai" are also both brands of dairy products in Japan, and a whole chapter is centered around drinking milk.
Mistaken Nationality: Based on her appearance, strangers constantly assume Yotsuba to be a foreigner, and indeed she was born in another, unknown country. As a result, some students at Fuuka's school try to speak to her in English because all gaijin are American.
Parental Abandonment: We know nothing about her origins except that Koiwai found her somewhere overseas, which Yotsuba claims was "an island 'to the left'".
Sunny Sunflower Disposition: She's such a Genki Girl, she appears with a bouquet of sunflowers in the cover of the 1st volume. A later chapter even sees her fascinated by sunflowers in Jumbo's shop, and wanting to buy some.
Super Drowning Skills: Suprisingly averted, Yotsuba can swim quite well. Koiwai, Jumbo and Fuuka on the other hand...
Third-Person Person: Childishly, in the original Japanese (mimicked in the Yen Press translations); she combines this with an innocent imitation of her father's masculine language — the effect is hysterically funny but is, alas, Lost in Translation.
Trademark Favorite Food: As expected for a five-year-old, Yotsuba really likes ice cream. She also has a fascination for cup ramen. Curry is another favorite — one of the songs on the Image Album is called "Eating Curry (Very Delicious)".
One of the Kids: Can be this sometimes when friends are over or when playing with Yotsuba.
Reasonable Authority Figure: He's quite willing to tolerate, if not participate in, Yotsuba's sillier antics. On those occasions when she does actively misbehave, the hammer comes down hard — sometimes overlapping with Scare 'Em Straight.
Generation Xerox: Takes after Mr. Ayase, notably his romanticism and love of puns (much to Asagi's chagrin).
Generic Cuteness: Often mocked for either her plain looks, eyebrows, or pudginess compared to her older sister Asagi. Doesn't stop her from being popular with her classmates though.
Kind of averted in that she is actually regarded as being fairly attractive. For instance, Jumbo's initial reaction to her was to exclaim that she's a bishoujo, only to drop the "Bi" when he meets Asagi because...well, it's Asagi.