Thacia is Daniel's love interest; she is a young woman around his age, a pupil of Jesus, and a person with profound empathy for those around her. At first, Daniel doesn't understand his feelings, as he believes his life only has a single purpose: the pursuit of revenge. Slowly, though, he realizes that his love for Thacia and by extension his friends and family is more important and more worthwhile than his hatred of the Romans. Through knowing each other, they are both altered to stronger individuals.

He heard Thacia catch her breath, and turned and looked into her eyes. He knew he was not worthy of the gift he saw there, but he knew that at last he was free to offer her all that he had in return. In that one brief look they made a new vow together.
(Speare, The Bronze Bow, Google Books)

When they finally come to peace with their feelings, it is the culmination of their sometimes conflicted relationship. Thacia simply wants Daniel to accept love into his heart, for her and for Jesus, but really for anyone at all. Daniel wishes that Thacia shared his drive for revenge, but realizes that he wouldn't love her if she changed. In the end, Daniel's acceptence of love is what brings them together, not his actions, which more often drives them apart.