Due to the different way time worked in Faerie, Ash had aged to around thirteen mortal years of age by 2012. At one point, he was taken from his mother by the Unseelie King and taken to his Court. With his royal faerie blood, his Shadowhunter blood, and demon blood, the King saw in him a potential to be a weapon against the Nephilim. The King primed him with his blood and spells into be what he had envisioned for his First Heir. The King also took a bit of his blood, taken from a cut he made on his throat, and wore it around his neck to draw power from.
His mother often bargained with the King for his return.
Ash was in the weapons room when a girl, whom he would later learn was Drusilla, appeared out of nowhere. The sight of the stranger made him wary, and he reached for his dagger and demanded that she identify herself, asking if she was a Shadowhunter. When Dru called him out for being rude, he told her his name but continued to interrogate her, immediately asking if she had been sent by his mother and if she was worried about him. Their conversation was cut short when Dru was pulled back to the mortal realm.
As part of the magic the King had woven into him, Ash had an effect on people that made them want to follow and protect him. This affected Annabel Blackthorn immensely and the woman clung to him as his guardian in the Unseelie Court, using the King's desire for the Black Volume of the Dead in her possession as a bargaining chip. The King planned to utilize spells in the book to imbue Ash with powers, some of which would weaken the Shadowhunters more than Thule's blight. Ash was also aware of his father's history with the Nephilim, so when Emma Carstairs and Julian Blackthorn came claiming to rescue him, he resisted.
When the Shadowhunters and one of the King's sons, Kieran, were captured, the King ordered Ash to execute Kieran. Ash asked for a reason, and the King only supplied that he needed to spill royal blood as soon as possible, so that the magic of the Dark Artifices could start its work on him while he was still young. Ash hesitated but was urged on by the King, but before he could try, his mother arrived. Annabel, desperate to protect him and keep him away from their power-hungry plots, took him through the Portal leading to Thule.
Annabel went straight to Thule's Sebastian with Ash, hoping he would protect them there, in the world where Sebastian had won the Dark War, with the pair arriving at Thule around two years after the war. It worked for at least five years. Sebastian kept his son around, teaching him his cruel ways, and Ash passively watched Sebastian wreak havoc on what was left of the world. He spent his time playing video games, all the while growing attached to that world's Jace Herondale. Eventually, however, when Ash looked around seventeen, Annabel realized that Sebastian was growing weak. She feared that Sebastian would someday turn on them and kill Ash to regain his strength, and thus made a deal with Julian and Emma to save Ash.
At a nightclub base used by his father, Ash found a drawing of Drusilla left behind by the Julian Blackthorn from his world and, curious about the girl, kept it for himself.
Ash was with Sebastian when he and his army confronted the rebels outside the Silent City. He silently signaled to Emma and Julian that Annabel had not betrayed them. When a battle broke out, Ash was held at gunpoint by a rebel, the werewolf Bat, and simply convinced the rebel with his words to let him go. Acknowledging that Sebastian did intend to kill him someday, Ash joined the battle in his winged form, struck Sebastian and, with a command, disarmed him, allowing Emma to kill him with the Mortal Sword, adding that he did it because he liked what she told Sebastian about their world's Clary Fairchild. Annabel tried to pull him through the Portal back to their world but Ash, even later when Emma and Julian called for him, refused to leave Jace.
Ash has short white-blond hair, a sharp face, pointed ears, and the typical Fairchild green eyes. He has a wide X-shaped scar on the side of his throat, cut by Arawn. Ash also had black angel wings, tipped with silver, on his back that he could hide and spread out at will. He bears a resemblance to both his parents and even his aunt, Clary.
During his time in the Unseelie Court, he wore rich clothes as well as a golden metal band on his head, signifying his royal lineage. As a child, Ash was thin, looking almost unhealthy. He grew into a tall young man with broad shoulders.