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?