Grandfather is a Mentor to Cassia and, as a symbol, a Wise Old Man. For one, he's wise and old :).
The Wise Old Man represents the personification of the spiritual principle, knowledge, reflection, insight, wisdom, cleverness, and appears when the hero is in hopeless and desperate situations and needs reflection or luck.
Grandfather is and does all these things for Cassia. Even after he's passed, she's always thinking about him and what he would think and how he'd want her to act.
Grandfather is also a Mentor because he gives Cassia advice and insight and help her in (and to start) her journey. He encourages her and gives her the feeling that she actually is brave and has courage.
"You have words of your own, Cassia," Grandfather says to me. "I have heard some of them, and they are beautiful. And you have already given me a gift by visiting so often. I still love this letter because it is from you. I don't want to hurt your feelings. I want you to trust your own words. Do you understand?"
"Cassia," he whispers. I am giving you something you won't understand, yet. But I think you will someday. You, more than the rest. And, remember. It's all right to wonder."
"I love you."
