When Kendra first sees Lena, she observes that:
Despite her white hair streaked with a few black strands, her face had an ageless quality. Her almond eyes were black as coffee, and her features suggested a hint of Asian ancestry. Short and slightly stooped, she retained an exotic beauty. (1.69)
Despite this sort of bewitching appearance, Lena's introduced as the housekeeper. Just like her appearance, though, at first her behavior seems a little odd to Kendra. For instance, Lena sits outside in the rain to garden, and fearlessly stands on stools to hang wind chimes. While none of this is super strange, when Kendra tells her to be careful, Lena's response certainly is. She answers:
"The day I'm too old to climb on a stool will be the day I throw myself off the roof." (6.11)
That seems a little extreme, right? Lena's really not thrilled with the effects of old age on her body though, saying, "I find it a frustrating, humbling, infuriating experience" (9.22). Which makes us wonder: How old is Lena actually? This answer is that she's at least over one hundred years old—which seems pretty unrealistic, until we learn that she's actually a naiad.
Or more like she used to be a naiad—until she left the water to save a mortal man, Patton Burgess, who was courting her. She married Patton and lived with him until he died, and even though she became mortal after leaving the pond, it's taken longer for her age to catch up with her.
After Patton died, Lena had a bunch of adventures, which she tells Kendra about:
"I traveled the world to see what it had to offer […] I set some swimming records in Britain […] I worked as a painter, a chef, a geisha, a trapeze artist, a nurse. Many men pursued me, but I never loved again. Eventually, there was a sameness to the traveling, so I returned home, to the place my heart never left." (6.36)
It's a good thing she returned too, especially for Kendra and Seth since she's pretty darn nice to them, and works some wonders in the kitchen.
On Midsummer Eve, Lena is abducted along with Grandpa. But when Kendra comes to the rescue with a bunch of fairies, the fairies decide to return Lena to the naiad pond, thereby reverting her to naiad status and making her lose her human memories. Naiad-Lena isn't really bothered by the loss, since she doesn't remember having human memories in the first place, but Kendra's upset on her behalf. Will Lena remain a naiad forever? We don't know.