The Elemental's Guardian - The Elementals Trilogy, #1

Everything changed the day he ran into her with his jet thousands of feet in the sky. Freelance pilot Ferenc Janos's life is turned upside down when Olivia Gillies, a woman who can control air and travel through lightning, tells him he's the guardian of the air elemental. Around the same time as her sudden arrival, masked figures have been appearing around the city with the ability to summon and control strange creatures.
Their target? The homeless girl who happened to steal his wallet. Ferenc doesn't understand his new role or why he feels a strange need to protect her, but if he can't set aside his skepticism long enough to learn from Olivia, then a much more powerful threat could be unleashed onto the universe.