Meliana is the Daughter of the First Progenitor named Shika Madou or otherwise known as Shikama Dōji she also has a brother name Michaela and when she saw Mikaela/Mika in the Hyakuya orphanage she wanted to stay and have him be her twin big brother while her real one was asleep. You see she wanted Mika to be her twin till her real one woken up so when her real twin saw him they would be triplets.