What are the motifs in The Robe by Lloyd C. Douglas?

Coming of age is a recurring idea. Several characters mature over the course of the story. One of those is Demetrius. Demetrius is an angry young man when he's purchased as a gift for Marcellus. Over time, he comes to accept his role as a slave and it's noted that he always puts Marcellus above himself and that he is willing to lay down his life for Marcellus. While he remains conscious of his role as a slave, he also seems to harbor some desire toward Marcellus' sister, Lucia. When Marcellus prepares to leave for Minoa, Demetrius doesn't hesitate to say that he will go with Marcellus. Before they leave, Demetrius gives Lucia a ring that seems to be a symbol of Demetrius himself. He asks her to keep it safe and becomes so bold as to slip the ring on her hand. Demetrius believes that slavery is wrong but believes that he has been treated well.