Elda(Chi) was the younger sister made to make Freya happy again. Freya fell in love with her father/creator Ichiro Mihara. Then Elda(Chi) asked to keep Freya's heart in hers, but lost all her data. That was when Hideki found her.
Freya became depressed after falling in love with her father at bottling up her emotions. Elda was created to keep her company, but the bond was not strong enough. Although Freya loved Elda, Freya was self destructing. Elda realized her sister's ailing heart and asked to keep it in order to keep her sister's data. Elda was overwhelmed and lost her data and their mother plotted to make Elda happy again.
Chi is unaware of Freya's existence until the persocom incident. Freya keeps telling Chi that she must find her important person and hacks her sister to assure it happens.