Entwined Fates: The Key of Fire by Juliane Maibach

An inheritance from her great aunt turns Teresa’s life upside-down. She moves to San Francisco and has to find her feet in a new city. She does her best to avoid the school heartthrob Ayden, who has a far too magnetic effect on her. Meanwhile, she realizes there’s something strange going on in her great aunt’s house.

She finds a mysterious key that opens a door to another world. When she returns home a short while later, she’s not alone. From then on, she has a constant companion – a small fox that appears to possess magical powers.
Nor can she seem to shake off Ayden. She finds herself falling for his mysterious charisma and tries to get a glimpse behind his façade. And he seems to take an interest in her too – until one fateful day when everything changes.