Here are some more answers to asked questions:
Does Kiel have secrets Elaru doesn't know?
Of course. Elaru isn't omnipotent. She didn't even buy a D.I. investigation report on Kiel, so certainly there are things she doesn't know. If we ignore all those little secrets every person has, and concentrate only on the big secrets, then, yes, there are a few.
I've already given you certain clues to speculate about regarding some of his secrets. In fact, in the next Aethernea book, we will get to understand Kiel's past and how he came to be who he is today. There is a rather interesting secret involved in this that will even cause Elaru to be shocked speechless. In fact, when Kiel finally tells her about it, she will display a rather strange behavior that will puzzle Kiel for some time.
Other than the secrets pertaining to Kiel's past, there are also several secrets about himself that even he doesn't know, let alone everyone else. But these things will not pop up for quite some time. At earliest, perhaps they will be mentioned in the third book.
Is it a thing for siblings to call each other “brother”? Why don't they call each other by their names?
Rhur calls Kiel "little brother" to emphasize how Kiel is "little" as in lower in status than him. Kiel calls him "older brother" to emphasize that he is just "older" but not "bigger" or higher in status as they are considered the members of the same generation and thus technically equal.
It is considered rude to refer to one’s parents, grandparents, uncles, aunts, etc.(the older generation) by their name so nobles refer to them as grandfather/uncle/etc. However, even when referring to the younger generation, or anyone really, nobles tend to often use their family relation to refer to their family members to emphasize on status or to show affection.
How dark will Aethernea go?
Not very dark. It will have some relatively mature themes such as mild sexual stuff, possessive and obsessive behavior, fighting and shedding blood, etc. There will also be some drama and angst, perhaps betrayal, some people might die, ...
You can expect an emotional roller-coaster at times, but overall, the story isn't depressing or tragic and doesn't contain horror. It is supposed to be a story that will make you happy, not make you depressed or sad over it.
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