We meet Nia for the first time in the 9th episode of the series.
We soon find out what kind of personality she has: she's very quiet and curious. The second adjective fits very well with her, since she doesn't know many things about the world (she doesn't even know the meaning of words) and she frequently ask them to whoever is in front of her.
She is found by Simon when he's going to give up in a fight after Kamina's death. Her body is inside a box thrown away from someone. When she wakes up, she starts to talk with Simon and asks him any kind of things, but then they will be attacked by a Gunman, but they will be safe thanks to their cooperation. Nia will be introduced in the Dai Gurren's team and, when all them find out about her real rank (she's the princess of Teppelin, daughter of Lordgenome), they will think to use her as hostage. Unluckily, all of them will soon find out that Nia has been thrown away from her same father and she's totally useless for her family. From that moment on, all the team will welcome her happily and will protect her.
Even if she's very quiet and cute, she's a very direct person, who says everything she thinks when she wants. She always try to stay near the persons she loves and to cheer them up, especially with Simon. At first she has not a great moment with Yoko, but after that this last one will know her better, she will start to appreciate her.
Seven years after the "death" of Lorgenome, Nia lives in Kamina City with her companions, and especially, with her Simon.♥ He decides to marry her, but she refuses, because she's not the kind of person that thinks that a person can become one with another. After that she will talk with Kiyoh and Kiyal, she understands that Simon is the only person she wants to be with for the rest of her life, and she will accept his proposal.
Right after this, Nia's true identity is revealed: she's the Messenger of the Anti-Spirals, a population that is against the earth and all the Spiral's supporters. Because of this, she will be separated from Simon and all the others for a long time, when she will be judged as evil. But still, the ring that she has in her hand, will lead her to continue her real goal: becoming Simon's wife and be happy with all her friends. She will be through the mill in this period, but she will constantly give advices (even if they can't seem like this) to her companions.
In the last part of the series, she will be again in her right mind and will be rescued from Simon in the last episode of the series. She will support him 'till the end, when, even if they both know that she will die, they will get married. Unfortunately, after the kiss, Nia will disappear, as she had not a human body... but her personality helped the most of the characters that were all fond of her. In fact, she will die with the smile upon her face.