Souko is the only daughter of the large financial group
Hoshina family. She got married to Aruto Tsukiyomi, despite the strong opposition. After Aruto disappeared, she held on to her belief that he would return. In Chapter 43, it is revealed that she never actually married Kazuomi Hoshina, since she never actually divorced her first husband, and still believed in Aruto.
In chapter 4 of Shugo Chara! Encore!, she is seen at Yuu and Yukari's wedding, then Aruto appears behind her and embraces her looking upon the crowd, though he signals to her to keep it a secret.