Three theories for why nya wasn't added to the team earlier: 1 since water is not one of the fsm's elements, it would unbalance the team dynamic. 2 because as we see she is two years younger than Kai, she was most likely considered too young to join at such an early stage. 3: it was to keep her safe, and for wu to not give any more clues about Kai and Nyas parents.
If you look closely at the Anacondrai spell, you can see that he only needed certain elements, and water was not one of them. He invited everyone to avoid suspicion and so that when he stole everyone's powers, no one could stop him. He didn't know if Nya had her powers at that time, so she wasn't invited.
Oh my gosh! I have to tell you, it’s completely unrelated to this episode, but I just moved into college and my faculty president legit says drAgon. Thats the only thing I remember from his whole speech
