In the beginning of the series, Yoshino is a girl who was very ill and cared for constantly by her grande soeur, Rei. Yoshino, however, is very independent and in some ways tomboyish, despite her sickly status and feminine appearance.

After her heart surgery, she becomes more energetic through out the series.
In addition to being Rei's petite soeur, Yoshino is also her cousin and next-door neighbor. Theirs is a somewhat rocky relationship, since Yoshino is eager to spread her wings, whereas Rei would like to protect her from the world's hardships.

In the first season, Yoshino is Rosa Foetida en bouton petite soeur, her grande soeur being Hasekura Rei.

In the second season, Yoshino becomes Rosa Foetida en bouton. She has no petite soeur yet.
(Source: Wikipedia)