Nika knew Uriel offered his heart as a sacrifice, but she didn't know Water warlock had done the same thing millennia ago. Water warlock kept talking about how water is the element of life and the amazing powers that he was able to conjure with just a thought but now that Nika suspected what was going on, she noticed how Water warlock couldn't keep himself from speaking about the things he liked just like Uriel used to do. The similarities were too many to be ignored and faithful to her style, Nika decided to confront the ancient being in front of her.
"I just confirmed it, you're possessing Uriel."
"You noticed... what gave it away?"
"It's impossible not to notice. He was such a charmer before Chichén Itzá, but now he's kind of an ass." She was careful to omit that the true reason she figured it out was that teasing Water warlock was as fun as teasing Uriel used to be.