Yotsuba Nakano is the fourth sister of the Nakano Quintuplets. She has a bright and cheerful personality. Yotsuba is the only sister who accepts Fuutarou since the start and wholeheartedly supports him in everything.

It is revealed in chapter 89 that Yotsuba was the quintuplet Fuutarou first met in Kyoto
(Source: Gotoubun no Hanayome Wiki)