Rachel is the daughter of Arlene Grace and Zahard.
Hear me out, alright? So we both know Zahard officially has no true blooded children because his power is too great and would result in the child or mother of the child becoming weak and dying. Basically, it's explained that Zahard is far too powerful, and the Princesses exist to decide who he'll "mate" with. Of course, there's a bunch of questions and controversy about that too, but we'll talk about this later.
Arlene was strong enough to birth a child of Zahard. It's one of the reasons why he fell in love with her. We know that he loves power and even seeks out more power thinking it'll help him win her over. He becomes King of the Tower just to make her fall in love with him over V, so it's easy to assume she's rather strong if he thinks so highly of having power.
Not only that, but the name Grace itself is legendary, and Arlene was supposed to be one of the irregulars who'd become a Great Family Leader.
So with the understanding that Arlene was one of the few women who were strong enough to handle Zahard, we can figure out one of the reasons he fell in love and was almost addicted to her.
We also know that Zahard killed her child, born from Arlene and V, which is later explained to be Baam/Viole. Well, it's heavily implied.
The issue I'm having with this is that Arlene seemed to love her child, just as much as she loved V, so how could Zahard have killed the baby unless he physically took it from her? It's extremely dark, but I believe he found Arlene, stole her baby from her, killed it, and then raped her. He desired her after all, and he was capable of killing a child, I have no doubt he'd take the next step. This would also explain why Eurasia Enne Zahard went on a rampage. She was angry at not only Zahard using the Princesses to silence dissent but also upset at Zahard for the tragedy he caused to Lady Arlene, whom Eurasia Enn Zahard had just recently learned about. Zahard must have confirmed that he has a true daughter, meaning he has no need for a wife or a child and is merely after Arlene now, maybe he's also after Rachel, but she left him to fulfill her mothers' prophecy.
This would all explain why Rachel in [Season 2] Ep. 240 believe she's the child of the legend and will be the one to end Zahard's madness and find the beautiful sky her mother desired. She is the child of Arlene, but she's a bastard. She's the child of hate, the child of suffering. It would also explain her attitude and disposition. We can see in Game of Thrones, children of hate, incest, and sufferings tend to be hateful and more sinister. Rachel truly thinks she's the child of the legend, and she hates those who oppose her, and deny her birthright. She doesn't know of Baam's true heritage, and only knows that Arlene and Viole are connected by some lunatic string of events, thus she uses him to achieve her goals.
I actually think she believes Viole is the one who brings her the Red Thorn to kill Zahard, which in truth is actually Enryu's job for Viole. She's so self-absorbed and single-minded she can't see the obvious because it's all she's ever known. All she knows is the beautiful sky her mother might've told her about as a baby. In fact, my theory goes one step beyond that.
Arlene gave birth to Rachel before Viole came back to life in the cave and simply left her behind with the God to oversee the cave. It would explain why Rachel was the only one who had access when all others, the entire universe was cut off. Rachel was waiting for the prophecy. She was waiting to gain access to the Tower through that monstrous child locked inside the cave. Viole WAS her Enryu. The monster who'd deliver her a weapon to slay Zahard.