SPARK by Atthys J Gage

People are dying downtown, their bodies shriveled away to almost nothing.
The police are mystified, and outrageous rumors are flying: flesh-eating bacteria? an experimental bio-weapon? mutant mosquitos? Fifteen year-old Francy Macmillan listens, but says nothing. It isn't a comfort knowing that no matter how far-fetched the theories, the truth is even stranger.

For Francy, that truth wasn't very hard to find. It followed her home from basketball practice one night, a floating bauble of light that speaks inside her mind and shares her thoughts and her feelings. Is it an alien wanderer fallen from some distant star? Or a shard of some divine entity? Whatever it is, it seems to like her. She calls him Spark.

But as their friendship grows, a disturbing fact emerges: Spark knows who is responsible for those strange deaths, and it is up to Francy to try and stop them. Spark leads Francy into a bizarre alternate reality, along with her friends: beautiful Echo with the baby dragon tattoo; moody Brooke with the wicked jaw; and Owen Owens, the boy with the fascinating eyes who may just get around to kissing her one of these days—assuming the world doesn't end first.