The Bronze Bow is a 1962 Biblical novel by Elizabeth George Speare. It is a heavily religious book that includes themes of love and redemption, and winner of the Newberry Medal.
The protagonist is Daniel, a young boy who watched his parents crucified by the Romans. His sister, Leah, also watched, and was so devastated by the experience that she became mentally ill -- "possessed by demons." Because of this, she withdraws and never leaves her grandmother's house. Later in the book, while Daniel is plotting with insurgents to fight the Romans, he meets with two wealthy teenagers, Joel and Malthace (Thacia). Thacia becomes Daniel's love interest, and comes to care for Leah; she is the first person to see Leah's face, as the other villagers are scared of the "demon-possessed" girl.
[Daniel] could think of nothing at all to say, could only stand stupidly. How had she managed it, when no one, not a neighbor or an old friend, had been allowed to see Leah's face for almost ten years?
"We've been having a lovely visit," Thacia said....
(Speare, The Bronze Bow, Google Books)
Thacia's concern and friendship help Leah begin to heal from her mental trauma.