I agree with Ulan-tan’s point of view, personally. We’ve seen a lot that proves his theory of symmetry, and I think that it also provides the best ending to the franchise, one where we give up our Light to end the Darkness.
As for the feminine terms used for the Light and Darkness, you’re overthinking it. It’s not uncommon to use female pronouns for things like ships, guns, machines, etc.
English
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Also don’t the Traveler nurtures, so it makes sense to me to for the gender to be female.
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<But the Deep is also an entity... What is it?>
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Not every entity needs a sex. If the Darkness really is a God, possibly as old as the universe itself, it wouldn’t need to reproduce.
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<Not saying it needed to...>