Ixtel's a 17-year-old Mexican-American girl who's had a seriously tragic life. Here's a brief rundown: at 14, her face was horribly disfigured in the car accident that killed her parents. After the accident, she became addicted to drugs, dropped out of school, and started working as a prostitute. So when Marcelo meets her at the Sisters of Mercy convent, where she lives with 39 other girls in what was originally a house for one rich family, her life's actually on the upswing. She may still be missing part of her face, but at least she's sober and back in school.
Although we don't actually meet Ixtel until the end of the book, and then only for half a chapter, she's one of the most important characters in the story. If he hadn't found her picture, Marcelo would have put in his summer at the law firm and gone back to Paterson. Instead, Ixtel became the first person with whom Marcelo truly empathized, and in helping her, the world got way more real than he ever imagined.
When Ixtel and Marcelo meet, he asks her how she was able to make peace with what had happened to her. She says she learned to forgive herself for the bad choices she made after the accident: "It's crazy, isn't it? To have one part of your self be nice to another part. Like the nice part of my face saying nice things to the ugly part. After a while, the nice part and the ugly part stopped hating each other" (29.74).
So what does this mean for Marcelo? Well, when Ixtel asks him if he has any ugly parts, he asks, "Is not seeing any ugly parts in myself an ugly part? Is not wanting to forgive someone's ugly parts an ugly part in oneself?" (29.80). He's talking, of course, about Arturo. And even though Ixtel tells him, "I didn't understand a word you said" (29.81), she's just brought about a major change in Marcelo by pointing out one of those gray areas Rabbi Heschel has been trying to teach him about.
In other words, the change Ixtel brings about in Marcelo is an understanding of the need to have compassion for the sinners, not just the saints. It's easy to feel empathy for a disfigured kid; it's harder to feel it for someone like Arturo. But Marcelo sees that it's important to love someone despite his flaws; despite the bad choices he makes. In teaching him about the importance of forgiveness, Ixtel gives Marcelo a gift that his religious studies could never provide—she teaches him how to apply the lessons of scripture to the (wait for it) real world.