Caius
character in the book "The Valiant" by Lesley Livingston
Kay Faraday
main character in Ace Attorney spinoff, 'Ace Attorney Investigations'
Sir Kay (sometimes Key)
King Arthur's foster brother and one of the first Knights of the Round Table
Kai (or Kay)
character from Hans Christian Andersen's fairy tale "The Snow Queen"
Lia Overbrook
main character in Wintergirls by Laurie Halse Anderson
Jezelia "Lia" of Morrighan
main character in the book series "The Remnant Chronicles" by Mary E. Pearson