Lucatiel is from the land of Mirrah, a place where only those who can fight are honored. She was born into the household of a peasant, and along with her brother rose to a respectable rank of knighthood. She and her brother maintained a rivalry throughout their lives, but she admits that out of all the times they fought, she never bested him once. Lucatiel and her brother distinguished themselves on the battlefield, as Mirrah is constantly beset by conflict, and were knighted into Mirrah's official order.
Upon attaining knighthood in Mirrah's official order, Lucatiel was issued with her sword. The official Dark Souls 2 guidebook indicates that Aslatiel of Mirrah, a red phantom fought in Aldia's Keep, is her brother. They both carry a very similar weapon, so it can be assumed that her brother became a knight of the official order with her.
Lucatiel's brother became afflicted with the undead curse. Instead of telling anyone where he was going, he simply left Mirrah one night and was never heard from again. Later on Lucatiel too became afflicted and traveled to Drangleic. She indicates that she came for 2 reasons:
1. To claim her share of the powerful souls that fill the land, and thereby skirt the fate of the curse.
2. To find out what happened to her brother.
As time goes on she becomes hollower until we meet her at the keep of Aldia outside the entrance. If we have fought with her by our side she will give us her armor, weapon and shield, losing what makes her a knight of Mirrah.
It would seem that by the time she has reached Aldia's Keep, she has become convinced she cannot escape the curse's fate. She then finds out what happened to her brother, that he forsook his knighthood to prey upon others, that the curse drove him to hollowing. Accepting this, she herself gives up, and allows the light to fade from her eyes.