He is a main character in The Lost Prince.
In order to be with Meghan in the Iron Realm, which is poisonous to Winter and Summer fey, Ash travels to the end of the world to gain a soul and become human. Before he can attain a mortal form, however, he must go through several trials, one of which is to realize what it means to be mortal. In other words, to cope with aging. To accomplish this, he sees a vision of the future in which Keirran is first featured.
Meghan, now the Iron Queen, realizes that she is pregnant and is shown a vision of her future son by the Oracle. In her vision, she sees her son standing over the dead body of her brother, Ethan Chase, saying he is sorry. Devastated by the news that her son is destined to kill her brother and cause the downfall of the entire Nevernever, Meghan decides to never let her son meet her human family. At the end of the novel, Ash names their unborn child Keirran.
Attacked by a new kind of faery, Ethan Chase (now 17) and his classmate, Kenzie, flee into the Nevernever and travel to the Iron Realm to find Meghan. Even though she tells her brother to stay in the castle, Keirran ends up finding him and helping him escape. Keirran is shown to be strangely sympathetic to the new faeries, the Forgotten, because his girlfriend, Annwyl, is also exiled and fading.
Annwyl, Ethan and Kenzie go to the New Orleans goblin market to find the Iron Prince, who vanished without a trace.
Kierran here, plays as a traitor throughout the whole book. He sides with the Forgotten Queen, who wages a war on The Courts. Kierran acts as a peace negotiator, which is the first time he is seen by Ethan Chase. Later on, you find out, that Kierran's soul can return to him, is, if he destroys the amulet that Awylen bears. Kierran does indeed change throughout the book, as you see that his personality has a drastic change throughout the story. He is, without his knowledge, the Lady's secret weapon, for it is profetizized that it shall be him who will unite The Courts, or destroy them. Kierran plays a small part of the book, until the end. You see clearly in the book, that his loyalty not fully firm towards the Lady, by the encounters he has with his parents and when Ethan Chase or McKenzie St.James mention Awyln's name, who he parts ways when he joins The Lady
Keirran was very troublesome as a child. He is also a loyal and protective son, with a streak of humor and mischief. He is also troubled and easily swayed, the human part of him causing this.
Keirran is described as a handsome, tall, and lean young man, with long, silver hair and bangs, and icy blue eyes like his mother. He has the typical sharp angled-face and pointed ears of a fey, and the inhuman beauty. He stopped growing when he was 17. He is half human but still has Summer, Winter and Iron glamour.
Keirran formed a relationship with a Summer sidhe from Arcadia, Annwyl. She is described as willowy with chestnut brown hair, emerald green eyes and skin like buttermilk.