A Pinch of Moonlight

Demi-Lee Jenkins is a British teen whose only real talents are stubbornness, whistling, and a hint of magical ability. Blackbird is the rogue male fairy who adopted Demi as his 'little friend' when she was seven. A Pinch of Moonlight tells their intertwined stories, as they find themselves responsible for the fate of the magical realm of Annwn.
In unjust exile from Annwn, Blackbird is searching for Demi-lee and encounters her at midnight on the summer solstice. In the company of two friends, Vicky and Heledd, Demi-Lee accidentally cast her first spell and awakens the spirit of Aelwen, her witchy distant ancestor. Blackbird uses the situation to worm his way back into Demi's life, hiding the fact that he's in exile from Annwn and has lost all his magic powers. Blackbird and Aelwen offer to mentor Demi and develop her magic powers.
Heledd has just finished a degree in ancient Welsh literature, and already has expertise in myths and legends, so she's thrilled to have magical beings to help her with her research. Demi's cousin Vicky is less thrilled - as a sceptic with a degree in IT, it's a real shock to discover the world is less rational than she thought. Vicky is deeply distrustful of Blackbird, and when she saves him from hypothermia, he reveals the truth of his situation.
There is discrimination against fairies in Annwn, and Tefyn, the elfish First Citizen, shocked many when he took the fairy Pefryn (Blackbird's cousin) as his wife. Hafren, Chancellor of Annwn, loathes all fairies, but especially those such as Blackbird or Pefryn who aim for a Place in society.
Hafren manipulated Tefyn into going on a quest to earth, then made Tefyn believe Pefryn had cheated on him with Blackbird. During the ensuing fight, Tefyn broke Blackbird's wings and Blackbird permanently injured Tefyn with a blast of magic. Tefyn had Blackbird's wings removed with iron, neutralising the fairy's magic.
Although Tefyn soon realised Blackbird was innocent, it's high treason for a fairy to injure an elf, so Blackbird was exiled. However, there's a chance that Blackbird can make things right, if he can discover the magic to heal Tefyn. In Annwn, a damaged man cannot rule, and Hafren is using Tefyn's disability in an attempt to seize power.
The humans, Aelwen (now in a hybrid human/plant body) and Blackbird work together to find a spell to allow Blackbird to return to Annwn, and another to let him and Tefyn heal each other. The ritual is performed at the next full moon, but it doesn't work until Demi-Lee goes with him, to supply the essential spark of magic.
Once in Annwn, Blackbird is reunited with Pefryn, who is in internal exile, but he's soon captured by his enemies and left tied up and beaten outside the city gates. Pefryn takes Demi-Lee to rescue him, but Tefyn has got there first and taken Blackbird to the healers. Hafren is lying in wait for Demi-Lee and tricks her into Walking the Walk, a dangerous right of passage for which she's totally unprepared. Demi could just leave Annwn the way she came, but the nixies will disown her and Blackbird, leaving them trapped forever in their respective realms, never to meet again.
Demi successfully walks the walk, but her magic now burns like nuclear fuel, and there's a risk she may explode. Tefyn is healed, and publicly reclaims Blackbird and Pefryn. Blackbird's magic is restored, and Hafren is not just thwarted but left almost powerless after a spell backfires on him. This isn't the end though - both Demi-Lee and Vicky have fallen for Blackbird, and Hafren's supporters are determined to avenge him. Volume two continues the story...