It's a strange way to ask the question by saying how characterization is "used" in a particular work of literature. It might be better to ask for characterization to be explained within a particular piece. That being said, Hannah Tinti sets her characterization among mystery in order to supply suspense for the readers.
First, let's take the character of Ren. We are presented with Ren's mystery right off the bat: Ren somehow lost his hand and his family, ending up at Saint Anthony's Orphanage. As the protagonist of this story, this twelve year old child learns about a life of thievery due to his "adoption" by Benjamin Nab and his partner, Tom. Why did they adopt Ren? They adopted Ren to further con people out of more cash due to sympathy. Ren is the "good thief" of the title in that he is able to see the best in the lowest of the low in society and have compassion for all. Near the end we are further presented with the mystery solved. Ren is not the boy's name, but his initials: Reginald Edward McGinty.
Next, let's look at Benjamin Nab. When Ren first "meets" Benjamin Nab near the beginning of the work, he claims to be Ren's long lost brother. Here we are presented with mystery in regards to characterization yet again. Who is Benjamin Nab? The actual answer we learn, as readers, throughout the story: Benjamin Nab is a thief. The mystery is furthered, though, in that Benjamin Nab "adopts" Ren, but is finally proved to be Ren's biological father. The final mystery of the characterization is the idea that Benjamin Nab, in his efforts to help Ren, just might be a "good thief" as well.
In conclusion, another aspect of characterization that is important are the scores of minor characters that show Ren's compassion. Look at his surrogate mother figure, Mrs. Sands. There is Jenny, otherwise known as "harelip." Further, there are the twins who are Ren's best friends, but considered bad luck to everyone else.