[ The mention that his brother shared his same abilities is all the explanation Zelda feels she needs to understand why the monster his ghost transformed into would be a danger. Even though the spirits were not actually the people they took the shape of, they somehow mimicked their abilities. A monster possessing Darin's abilities as well as the ability to siphon someone's vitality would be a formidable foe.

No, what catches Zelda's attention is how Darin insists that he is responsible for his brother's death. That... That can't be right. It doesn't matter that she doesn't know the circumstances that lead to Dromas' death; there's no way Darin intentionally killed his own brother. They were only children. Whatever happened, whether an accident or something worse, the fault could not possibly lie with Darin-- with either of them. She refuses to entertain the thought that he was any less a victim than his brother.

However, Zelda keep this conclusion to herself as she quietly holds the end of the bandage in place. It's not her place to speak on something she knows nothing about and doubtless all she'll manage to do is upset Darin. ]

That soul was not truly his. [ She offers. ] It was... blank, originally. Somehow, you imprinted your memories of your brother onto it and that is why it took his form.

[ Rem said that was just a theory, but having had time to think about it, it makes the most sense. ]