Young adult horror novel series where a brainwashed girl turned out to be 100 years old and controlled by some ball lightning creatures

Question

I would have probably read this in the mid/late 1990s or very early 00s, but I'm trying to hunt down a novel I remembered earlier. I'm pretty sure it was part of a series. I was living in Ireland at the time, if that helps - I think this series could have been from the 80s, too, as I may have gotten the books from the library.

The basic plot of the first novel was that a strange girl appeared in school, and the group of friends/siblings/whatever found out she was possessed/brainwashed and was like a hundred years old, and being used as an agent for these weird will-o-wisp/ball lightning creatures, and in the end she was freed from their control.

I want to say the strange girl was a witch or something, and after she was freed, she joined the group of friends but had no memories of the modern era at all. I vaguely recall that she had some kind of powers they used to fight the ball lightning things, either at the end of the novel, or the end of the series maybe. I could be wrong, but it may have also been set in a seaside town, and one of the books may have been about a haunted house or something?

Answer 1

Just to put the answer out there, after I suggested that it sounded like a Christopher Pike novel (him having written several books involving people being possessed by members of an ancient race), the querent stated that they believe it is Spooksville #15: Revenge of the No-Ones.

Mysterious balls of light are circling Spooksville, and one of them zaps Adam--invading his mind. Worse yet, the balls of light are slowly moving toward town, preparing to turn everyone into strange alien creatures.

This review mentions that Tira Jones, newly introduced in the novel, had merged with the creatures in 1832 and is now an unaging witch, with all spiritual and magical powers coming from these aliens.

It seems that a million years ago some aliens came to Earth. No, not those ones, another group of aliens. But there was a betrayal and they all died, but because they were so far from home their souls got stuck here. So ever since: all the possessions and demonic presences you hear about through history? That’s these souls, trying to get back to a body and fighting with the soul already inside it. Tira was one of them, a lonely girl mentioned in Ann Templeton’s great-great-grandmother’s diary who just disappeared one day. But now she’s back, with whatever powers might have evolved over a century and a half of possession, ready to help the No-Ones become someone again.