Lyria was a fae and the former wife of Rowan Whitethorn.
Approximately 200 years before Heir of Fire, Lyria and Rowan Whitethorn were a couple, and believed to be mates. When Rowan was younger, he met Lyria while she was selling flowers in the markets of Doranelle, and immediately knew she was his mate.
They were together for a while and had a small house in the mountains of Doranelle. Although Rowan was happy with Lyria, mating her had cost him his good favour with Maeve, as Rowan was a prince of Doranelle and Lyria was seen to be beneath him.
Therefore, when war came and Maeve offered Rowan a chance to redeem himself, he jumped at the chance. He left Lyria at their home, not knowing that she was actually pregnant with his child.
Rowan was gone for months and eventually, they got word that enemies were secretly trying to enter Doranelle through the mountain passes. Rowan flew home as fast as he could, but it was too late - Lyria was slaughtered by enemy forces, along with her unborn child.
Following her death, Rowan went mad and wandered the world for a decade, completely lost. Eventually, he was approached by Maeve, who told him he had spent enough time grieving and offered him a chance to work for her. Rowan agreed, and after tattooing his grief and shame about what happened to Lyria onto his body, swore the blood oath to Queen Maeve.
Queen of Shadows
In Queen of Shadows, Rowan is reminded of his former mate when he sees a girl selling flowers in the markets of Rifthold. He becomes very sad and admits to Aelin that he knows he did not deserve Lyria.
Empire of Storms
In Empire of Storms, it is revealed by Maeve that Rowan and Lyria were not truly mates - she used her powers to alter fate and made both Rowan and Lyria think they were soul mates. However, Maeve tells Aelin that she is in fact Rowan's mate, and she used Lyria's death to break him, knowing that once Aelin was born and came to Doranelle, she would sacrifice anything to protect Rowan - including giving up the Wyrdkeys.
Kingdom of Ash
In Kingdom of Ash, Maeve induces a dream-like state for Rowan during the battle in which she makes him think Lyria is talking to him, and Lyria says, "You would pick [Aelin] over me?" Of course, this isn't real, but it does hurt Rowan emotionally and allows Maeve to manipulate his feelings.