Yotsuba Koiwai is a curious and energetic 5 year old girl!
she was adopted by Yousuke Koiwai who found her while abroad, she is terrified of bullseyes and owns a stuffed bear, Juralumin


her friends include:

Ena, Fuuka, Asagi, Miura, and Jumbo

PicotheSpicyWarlord explains why you should love Yotsuba too

Yotsuba and 4chan
sometimes referred to as "404 girl," some users of the site consider her to be the mascot of 4chan
(more info)
example of a 4chan 404 image including Yotsuba (despite 4chan's general reputation, threads/posts on Yotsuba are usually wholesome ^^)