It is revealed during the events of "Houkai 3rd", that Kiana Kaslana in this title is actually a clone of the real Kiana, also known as test subject K-423. She was created by Otto Apocalypse who implanted a fragment of Sirin (Houkai 3rd)'s core into her body. At some point she awakens as the Herrscher of the Void due to these experiments which is basically Sirin's consciousness taking over her body.
The "Herrscher of the Void" was defeated during the events of the "Final Lesson" chapter. Kiana reappears as "K-423" in the Ver. 3.3 Expansion and seems to have control over some of her powers as a Herrscher while retaining her personality.