For the sake of you playing in the future I won't go into the details.
little bit of spoilers, since everything is explained later in the game.
Yes, Rule B and Gab are the same person (that much is to be expected), and he's also the MC's older brother. Nael IS the strongest Dark One, and will not be part of the harem. Claire in't evil, she is just very cunning and tries to protect Clara. Your questions about the MC can't be answered yet because the game hasn't come that far yet.