Who is Lena in the novel, Fablehaven?

Lena is introduced as the housekeeper, but she is actually a former naiad who is hundreds of years old. She is kind to the children, and she tells Kendra the story of how she became human. She does not have any magical abilities because she is human, but she has lived for hundreds of years and probably has a lot of years remaining. She admits to Kendra that she has thought a lot about her mortality, especially in recent years, but that she would make the same decision again if she were faced with the opportunity to spend time with the human whom she loved.. Lena is turned back into a naiad as the novel comes to a conclusion, but Grandpa says he will look for a way to change her back.