NieA is everything that Mayuko is not. Mayuko is reserved; NieA is an extrovert to the extreme. Mayuko worries about being responsible and frugal, NieA often does whatever she feels like and damns the consequences. Mayuko tries to always take into account how others view her, while NieA couldn’t care less. Mayuko lets the pressures of life get her down, while NieA is always soldiering joyously on and relaxing under circumstances that might have Mayuko stressed to the breaking point.

NieA’s entire existence is something of mystery, even to herself. She exists on the fringes of both human and alien society, unwilling or unable to conform to the routines and expectations of either. It’s unclear how long she’s been living at the Enohana Bathhouse, as she claims she was living there when Mayuko moved into the room they share. She is at the bottom of alien society and will never be able to get anywhere else as she lacks the one feature that her kind seem to pride above all else—an antenna on the top of her head. The Japanese government doesn’t even acknowledge her existence due to her low caste status. She is termed worthless by most aliens who comment on her, yet she is the one with whom the mother-ship communicates as it is preparing to depart Earth.

While NieA remains playfully indifferent to all the scorn and disregard she faces, the mystery of the mother-ship and its communications trouble her. These “broadcasts” that only she can hear parallel the growing pressure that Mayuko is being subjected to and, eventually, NieA goes through her own transformation; she vanishes for a good portion of the summer—two episodes of the series—and while there doesn’t seem to be an obvious change in her personality, she is shown to be more articulate about and aware of her resistance to the structure of society that forces expectations upon people and forces individuals to conform to certain predetermined molds. She doesn’t seem consciously aware that anything has changed about her, or, if she is, she doesn’t make a big deal out of them. She also doesn’t seem to take notice of the fact that she continues to receive broadcasts from the now-vanished mother-ship. (NieA is shown entering a trance in Episode Thirteen that is similar to those who fell into once the mother-ship started communicating.)
Why was NieA the only one to hear the mother-ship? What did she discover during her absence? What is happening to her when she goes into her trances? No one knows, perhaps not even NieA. What is clear about NieA in the series, however, is that while Mayuko is the main character, NieA is definitely the story’s heart. When she vanishes mysteriously during Episodes Eleven and Twelve, her absence is felt as keenly by the viewer as it is by Mayuko; up to this point, NieA has seemed largely to be a source of comic relief and the primary manifestation of the aliens’ narrative role of underscoring the sorts of pressures that grind under people like Mayuko. Once she’s removed from the scene, however, it’s as though there’s a void in the series’ world. The viewer feels the same sort of loss that Mayuko comes to feel when she recognizes that NieA is actually missing, and it makes the observations by Genzo and Chiaki that there is something magical about NieA’s carefree existence and attitude.