She's 180 years old. Her innate ability that she's mastered is being able to become human so she can easily switch between the land and the sea.
While she did have a love for humans growing up, over time that soured and now she mostly enjoys human culture rather than the people themselves. She doesn't feel inferior around them and thus isn't crazy for their destruction like with the giant krakens. But that also means if humans get stuck in the crossfire she isn't going to do anything to help them.
She was kicked out by the other merfolk, which had been a long time coming truthfully, and was hoping to use the trident both to get rid of the giant krakens and reinstate herself as the mermaid queen by force. If a way is presented to her, she may consider turning Oceanside into a kingdom but even Nerissa herself finds the idea a little farfetched given the krakens are still kicking around. For now, it's just a silly little dream to think and ponder about.
Nerissa's good at singing, dancing, and most of all acting. These were things she picked up while jumping back and forth between land and sea. The theatre was one of her favorite things to watch when she could and it directly inspired her bombastic tone in the modern day.