Maybe there is a strong - willed female lead. She could be the one who has to steel her heart in a male - dominated world. For example, she might be facing discrimination or hardships in her career or personal life, and she has to toughen up. There could also be a mentor - like character who helps her along the way.
It's possible that there is a young hero in the novel. He has lost someone or something dear to him, and as a result, he decides to steel his heart. There could be antagonists who try to break his resolve, but he stands strong with the help of his friends. These friends could also be important main characters in the story as they contribute to his growth and the development of his'steeled heart'.