Kieran was born to the Unseelie King and a water faerie around 1992. Because of the faerie's agelessness and the different flow of time in Faerie, Kieran has lived much longer.
Though Kieran is an Unseelie Prince, he is one of the youngest and the least regarded by the King of among fifty. In the Court, however, Kieran was popular and loved. One of his half-brothers, Erec, and the King apparently saw this adoration as a threat, and, for this, his own father traded him to the Hunt at a young age.
As a Hunter, Kieran was given a fierce, black faerie steed by Gwyn. He was mocked by other Hunters for his status as a former prince, and he made no effort to make friends with them. Nonetheless, Kieran was intrigued by Mark Blackthorn, a fellow-shunned member for being half-Shadowhunter. One day, after other members of the Hunt tortured him, Kieran went to his aid and treated him. The two became friends, growing closer and eventually falling in love.
In August 2012, Mark was returned to his family at the Los Angeles Institute as a bargaining chip, to get them to discreetly investigate a series of murders that had taken several faerie victims. Mark was given the option to remain with his family if they fulfilled their end of the deal, much to Kieran's dismay. During this time, Kieran paid him several secret visits and even lent him his steed once.
During one of these visits, Kieran witnessed Mark and Cristina Rosales talking in the kitchen of the Institute and overheard Mark casually sharing Hunt secrets about their leader, Gwyn ap Nudd, during a somewhat intimate moment together. Kieran was led to believe by Iarlath that Mark would face his doom with the Shadowhunters; fearing this, he told Gwyn that Mark had divulged fey secrets to Cristina, thinking it would lead only to Mark's return to the Hunt. Unexpectedly, however, Iarlath decided that Mark would instead be severely punished with whipping. Mark's brother Julian, and later the boy's parabatai Emma, volunteered in his place. Mark became angry at Kieran for inadvertently causing his brother pain and broke up with him.
Despite this, Kieran continued to apologize, showing his regret by helping Mark's family investigate the case. It was ultimately Kieran who discovered that Iarlath was involved with the Guardian, helping them deduce the latter's identity, and went to help them save their youngest sibling, Tavvy, where he and Mark fought side by side.
Kieran was tortured, particularly by his sadistic brother Erec, for weeks before his scheduled execution at the Unseelie Court. To learn more about the Shadowhunters he'd interacted with, the King took his memories from the weeks prior, so when Kieran was rescued by Mark, Julian, Emma, and Cristina, he had no memory of being partially responsible for Emma and Julian's whipping, nor of Mark breaking up with him. In an attempt to protect Mark, Kieran took an arrow meant for him.
After taking refuge and recuperating from his injuries, healed by Mark's aunt Nene, at the Seelie Court, the group met with the Queen, who told them of her desire to work with the Nephilim against the King, in exchange for the Black Volume of the Dead and the end of the Cold Peace. Keeping the former a secret amongst them, the Queen appointed Kieran as her envoy to the Clave to relay her message. To prove his willingness to participate in the plan, Kieran attempted to swear loyalty to Mark but was stopped by Julian, saying his commitment should be free of entanglements. Kieran decided to swear fealty to Cristina instead.
Kieran initially decided to swear to Cristina to spite Mark, but soon found himself admiring Cristina. Kieran contacted Adaon, whom the Queen suggested could make a favorable succeeding Unseelie King, to involve him in their plan and ask him about a faerie binding spell that had plagued Mark and Cristina. Their clandestine meeting was interrupted by Erec, who ordered Kieran to kill Cristina in exchange for his help to regain the King's favor. At the last minute, Kieran turned on Erec and killed him.
For killing his supposed favorite son, and for being an escaped captive, the King grew more furious at him and answered a Projection call meant for Adaon. During the confrontation, with Cristina and Mark in the room, the King returned to him his memories in an attempt to show him the Nephilim's own deception. Though Kieran was at first infuriated and hurt by the turn of events, he, with Cristina's help, came to terms with everything that had occurred between them and decided to continue working with them, but also broke up with Mark.
When the day of the Council meeting where Kieran was supposed to speak on behalf of the Queen came, Cristina prepared a plan with her ex-boyfriend Diego in case things went awry—which they did after Annabel went mad. Cristina asked Diego to hide Kieran at the Scholomance during the heat of the events that she expected would follow.
Kieran is said to be beautiful—inhumanly so—and fierce. He has high cheekbones and wide-spaced eyes, the left black and the right a deep silver. He is pale and his body is lithe and graceful. As a prince of Faerie, Kieran wears battered white gauntlets.
His thick hair changes color—specifically colors of the sea, due in part to his nixie heritage—depending on his mood, from dark black when he is in a bad mood to a pale blue when he's happy, though he can also change them at will.
Skills and abilities
- Enhanced senses: Kieran has sharp hearing, a common aspect among full-blooded faeries, and a heightened sense of smell.
- Accelerated healing: Like all Hunters, Kieran is protected by magic that significantly hastens their healing process.
- Equitation: Kieran is a skilled horse rider.
- Chromokinesis: Kieran can change the color of his hair at will or based on his emotions.
- Faerie steed: Kieran's steed as a Hunter is Windspear, a black and skeletal steed—a "mount of the dead".
Kieran and Mark met in the Wild Hunt. Both found solace in each other as both were ostracized in the Hunt. When riding together, Kieran made their trips fun, showing Mark wonders that only the Fair Folk knew of. Eventually, they fell in love and Kieran gave Mark his first kiss. Though Kieran tried to encourage Mark to embrace his faerie nature, he neither mocked him and himself told Mark that he could admit and confide in him that he still felt like a Shadowhunter.
When the pair had a falling out after Mark was returned to his family and Kieran accidentally got his brother and friend in trouble, Kieran did not give up on him and instead decided to prove his love by continuing to help his family. Their relationship continued in some form when Kieran's memories of their breakup were taken by his father, but when he regained these memories, it was his turn to feel betrayed and used by Mark. After some convincing from Cristina, Kieran understood his motive and they cleared up the misunderstanding, but Kieran decided that after paying his debt to his family, they would be done with each other.
Kieran was at first threatened by Cristina and Mark's budding relationship. When Kieran needed to swear fealty to a Shadowhunter—that wasn't Mark, as insisted by Julian—for a mission from the Seelie Queen, he swore his fealty to Cristina instead. He did this spitefully at first, but he soon found himself respecting and admiring Cristina, seeing her as kind and trustworthy. Kieran sometimes calls Cristina "Princess",  first because of how she was introduced to him by mark and later as a sign of his respect for her. 
- Bitter of Tongue (first appearance)
- A History of Notable Shadowhunters & Denizens of the Downworld
- Lady Midnight
- Lord of Shadows
His first name is the Irish form of the Gaelic Ciarán (little dark one), which is from ciar (black, dark), a word meaning “dark-haired” or “black-haired” when applied to humans. 
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 Lord of Shadows
- ↑ Queen of Air and Darkness teasers
- ↑ 3.0 3.1 "His mom was a water faerie." Cassandra Clare on Twitter
- ↑ "Kieran’s other brother." Cassandra Clare on Twitter
- ↑ A History of Notable Shadowhunters & Denizens of the Downworld
- ↑ 6.0 6.1 6.2 6.3 6.4 6.5 Lady Midnight
- ↑ "... but [Kieran] never made it back. He was captured before he did," Cassandra Clare on Tumblr
- ↑ "Is there a reason why Kieran calls Cristina a princess" - Fan question answered Cassandra Clare on Tumblr
- ↑ "Origin of the name Kieran"-Cassandra Clare on Tumblr