Creating a visual representation of a character can be difficult. Good for you for undertaking this challenge!
When you read the book "An Ember in the Ashes" first look for hints from the author as to what Elias might look like. Perhaps the author mentions an item of clothing Elias wears or his hair or eye color.
Next, consider the character's age and maturity. Elias is young but he has faced many difficulties in his life that might make him look older than expected.
Make a note of key scenes in the book that you think really define Elias's character. What or who is important to him? What is he afraid of? What are his goals? How does he change throughout the book? These things can help you create a mental picture of the character.
A crucial piece in creating a visual representation of a character is considering your character's context. For example, it would be important in illustrating a historical novel to accurately represent the hairstyle and dress of the specified cultural time period.
Likewise, a character's occupation, social background, and economic standing all influence his or her visual representation. In "An Ember in the Ashes" Elias has trained to be an assassin at a military academy so you could research clothing styles that reflect his occupation.
Finally, don't be afraid to try many ideas until you find one that is a good fit for you. The process of creating multiple representations of one character is useful in developing your skills in visual art. Good luck!