@Hachikuji-San yes she is definitely some sort of ethereal being. We know the girl in the mask is Kaguya-hime like they talk about in 141, which must mean that Tsukasa is some sort of immortal moon maiden. She was never harmed at any point in the manga, despite being hit by a truck, she never got sick either. In the parents arc around ch.27, she was able to read an ancient script no one else could. So this means Nasa is going to die before her, somehow. Especially if they stay true to the myth. The manga will likely end with Nasa dying (hopefully) of old age and then Tsukasa returns to the moon, like Kaguya-hime does.