The girl discovers her magic and becomes the most powerful character at the end of the story. The boy and the girl have to hide from some kind of council which wants to destroy the girl(?). I think it is a multiple viewpoint novel, I listened to it between 2010-2016 on YouTube, it was a single book.
Like Moriarty, I was initially put off by the statement that this was a single book; otherwise, it sounds like a description of Mordant's Need by Stephen R. Donaldson. Mordant's Need was published in two volumes, The Mirror of her Dreams (1986) and A Man Rides Through (1987), but it really is a single novel, split in two. I realized that if you just heard a reading of the story on YouTube, you would not necessarily know that the story had been split for publication.
The story concerns Terisa Morgan a woman from our world who is pulled through a magical mirror into the land of Mordant. All magic in that world involves mirrors, and the success of the Kingdom of Mordant, which is trapped between two powerful empires, has been based on the fact that (almost) all the magically-trained master Imagers have been brought into the service of the Kingdom. Although the viewpoint follows Terisa most of the time, the story is occasionally told from other viewpoints (including the climax, which is told from the villain's viewpoint).
When Terisa is pulled through the mirror, their is a great deal of debate whether she actually existed before she arrived in Mordant, or whether she was created by Geraden, the man who pulled her through. (Donaldson tends to lard his writing with these kinds of metaphysical quandaries, although often the quandary is only a quandary if the reader accepts some other, equally dubious metaphysical assumptions that Donaldson apparently takes for granted.) While the reader appears to know that she was real before her summoning, ultimately the question is only resolved to the satisfaction of others near the very end of the story (unless you count an in-story joke about Geraden's inability to perform sexually as convincing).
Terisa and Geraden end up as lovers. (I believe the very last paragraph concerns their wedding.) Moreover, they also turn out to be extremely power Imagers, and at various points they are on the run from several factions, including Mordant's Congery of Imagers.
Wikipedia says, "This series is much lighter in tone than Donaldson's The Chronicles of Thomas Covenant, the Unbeliever or The Gap Cycle." I don't know what they are talking about. The books are quite brutal, with murder and rape; the climax of the book, although it has a certain ironic tone, takes place during another attempted rape.