Rowan and Aelin first meet in Wendlyn when he is given the orders from the immortal Queen Maeve to take Aelin to see her. Upon meeting in Mistward, Maeve gives him the order to oversee Aelin's training before she is allowed into Maeve's realm in Doranelle.
The relationship between Rowan and Aelin at the beginning of Heir of Fire is strained, to the point of getting into fistfights and beating each other to a pulp. However, after going through a lot together, such as fighting to get rid of Skinwalkers, and hunting for dead bodies which were washed up on shores, a slow understanding starts to develop between them both. They soon start building a relationship as friends and Aelin/Celaena even remarks that she 'had loathed him', because she felt that she was looking at a 'mirror image' of herself.
Rowan and Aelin's relationship grows even more, they become more than friends but less than lovers, when they become carranam, during the battle of Mistward, thus 'claiming' each other. Rowan even proves his complete loyalty to Aelin when he swears the blood oath to her, after Maeve freed him from her blood oath due to Aelin forcing her hand.
They have some parallels in their lives, such as their respective lovers being killed in gruesome manners and that 'collecting scars' was a result of self-inflicted 'punishment for (their) sins' due to their inability to protect others from themselves. They both share similar tattoos, Rowan's one stretching from the left side of his face to his torso; and Aelin's on her back, over her three main scars she received during her time as a slave in the Salt Mines of Endovier.
During his time in Rifthold, Rowan's relationship with Aelin grows very quickly. When they see each other after being months apart, Aelin embraces him and starts to cry, stating how much she had missed him. She later bathes him and contemplates about their relationship, how "the lines have always been blurred for them". And a few days after Aelin and Dorian shattered the Glass Castle Aelin and Rowan kiss for the first time.
After the battle in Eyllwe it is revealed that Maeve led Rowan to believe that Lyria was his mate, and then had her and their unborn child killed to break him. His real mate is Aelin, and they were married discreetly by the captain of the ship under the witness of Aedion and Lysandra. He then seeks to find a way to free her from Maeve.
Lyria was Rowan's believed mate. She was a flower seller in the markets of Doranelle. Their relationship, however, cost his favor with Maeve. When Maeve requested his presence to fight in an unnamed battle, he jumped at the chance to redeem himself, leaving Lyria in their mountain home, who unbeknownst to Rowan, was pregnant at the time. When he had been fighting for several months he heard rumors that the enemy was trying to reach Doranelle through a mountain pass. Upon hearing this news he returned home immediately, only to find her dead, murdered by soldiers. After he killed those responsible he went mad for ten years wandering the earth in his hawk form, and then Maeve found him and offered him a position among the Cadre. She has been dead 203 years, 27 days according to Rowan.
Maeve later reveals that she tricked Rowan into believing that Lyria was his mate and that the bond had been forced. It was Maeve who sent the troops through the mountain passage and ultimately killed Lyria and their unborn child.
When Aedion and Rowan first meet they are wary of each other, trading subtle comments and engaging in dominance battles. Aedion is eager to learn from Rowan and test himself against him, and is later furious when it is revealed that Rowan swore the blood oath, as he believed it was his by right. Rowan reveals that Gavriel is his father, much to the shock of Aedion.
The two eventually come to respect each other, with Aedion even referring to him as his brother. When Aelin faces the King of Adarlan, Aedion and Rowan battle the Wyrdhounds in the catacombs of the city and Aedion risks his life to protect an injured Rowan.
Rowan and Fenrys may have a confusing or unclear relationship, but Rowan may show a little affection. this can be proven by the fact what Rowan said to him when he took the blood oath and the nickname has given to him; "Boyo".