From Melissa Marr's website:
Niall (nye-ale) or (neil) -- "champion"; "passionate, vehement"; anglicized as "Neil."

The last Summer King accepted Niall into the Summer Court despite Niall's dubious past. His dual sense of gratitude and indebtedness, combined with his pre-Summer Court experiences, make him inflexible on ethics issues. Niall is a member of Keenan's Summer Court, and is one of his closest advisors. He's at least as old as Keenan, since he's been helping in the search for the Summer Queen since the beginning.

Aislinn, about Niall: 'The second faery had shorn wood-brown hair, and a face that would be somehow forgettable but for the long scar that ran from his temple to the corner of his mouth.'

Leslie says of Niall, "his face was as beautiful as Keenan’s but in a different way: Niall’s had character. One long scar ran from his temple to the corner of his mouth, and he wasn’t shy about it. His hair was cut so short that there was no chance of anything detracting from the beauty of that jagged line."

Niall has the sleek, musculature of a martial artist.

Niall seems the most empathic of Keenan's circle of guards, Summer Girls and advisors. He's aware of the impact the curse has on the court, noting it's effect on Tavish, for example. In addition, he is aware of the change the mortal girls undergo when they become Summer Girls. In a conversation between Keenan, Tavish and himself, Niall points out to Tavish mortal girls lose their inhibitions when they become Fey and that is why they are so receptive and "affectionate." Niall also knows how Donia feels, calling it "the Winter Girl's melancholy love."

The most important aspect of Niall's contribution to Keenan's decision-making is his concern with volition. Initially, this concern manifests when he carefully explains to Tavish why it is the mortal girls' attitudes toward sex and men change after they become Summer Girls.

Later, when Keenan has decided to take Tavish's advice and use whatever persuasion necessary to get Aislinn to accept her role as Summer Queen, Niall steps in and stops him. Moreover, in their discussion, Niall pushes Keenan to think about the sort of king he wants to be, now that he's on the brink of finally coming into his full power. Niall's advice is to be a fair and just King, setting an example and not allowing the type of cruelty the Dark court allows.

In Leslie's opinion, Niall seems sweet -- kind in a way that Keenan wasn’t.

Niall is one of the three POVs in the second novel, Ink Exchange.