Lucas grew up with the vampire, Anastasia. She flirted with him non-stop, enjoying the attention he gave her, and he gladly followed after her like a lovesick puppy. He fully believed that she loved him back, she just had trouble showing it. He hated James for trying to steal Ana's attention.
This continued on until Anastasia fell in love with a werewolf, and his accidental death sent her into a homicidal rage. Lucas knew in his heart that she had never loved him, but his devotion was absolute, and when she asked him to protect his sister, he obeyed.
Lucas had seen Clara many times before, but this was the first time he didn't have feelings for her sister. While protecting her, he fell in love with Clara, and she loved him back. Clara was hesitant, since Lucas had loved her sister for so long, and Lucas spent many months proving his devotion to her.
Ana returned from battle, a criminal amongst her kind, and went into a comatose state. With Clara gone one night, Ana woke up, and persuaded Lucas to sleep with her, using her powers on him so he would say yes. When Clara returned, she was destroyed, knowing Lucas had gone back to her sister so easily after swearing there was nothing between them.
Anastasia became with child, and Clara pulled further away from Lucas. Seeing her sister swell with the child of the man she loved was too much. The child was born, a girl, and Clara's mother sent for the Incubus Balthazar to take the child to the vampires. Ana's actions had put a knife over all their heads, and none of them wanted the baby to suffer for her crimes. The baby was named Elisabeth, and she was sent away, her lineage remaining a secret.
Though Lucas tried his best, Clara had not forgiven him before the three of them had to flee when Ana's bloodthirsty rages were discovered by the humans. The Countess took the blame, and sent her children away so they wouldn't be in danger. Lucas left the sisters but he and Clara kept in touch over the next four centuries.
Lucas' letters expressed regret, guilt, and love for Clara, but they could never be together with Ana's crimes hanging over them. The pain of losing his daughter and the woman he loved slowly made him go mad.
He has hinted he is the originator of the Phantom of the Opera legend, since he chose to live in dark hidden places in Europe, knowing it would keep Clara and Ana safe.
War of the Chosen
Bite of the Fallen
Rise of the Monsters
Time of the Ancients
Affairs of the Immortal: A Sinful Affair