Symbolism, Imagery, Allegory
Okay, we know—you're thinking, "Wait… isn't the Monkey King a character? What's he doing hanging out with the symbols?" But you see, in this book the major characters can serve as symbols and allegories too—the Monkey King is a good example of this double-duty work.
Think of the Monkey King's story as an allegorical tale of transformation. He embodies transformation—literally. By chapter two, he's mastered the "Four Major Disciplines of Bodily Form," like "Discipline One: Giant Form (this is a favorite of his), "Discipline Two: Miniature Form" (he's not so into this one), "Discipline Three: Hair-into-Clones" (we'd like to see this one ourselves; alas, he doesn't use it much), and lastly "Discipline Four: Shape Shift" (he uses this one when he turns into Chin-Kee) (4.9-4.12). That's a ton of transformation, right? Right.
Plus, his personal transformation is the first transformation we see: he changes from a violent, angry, self-absorbed monkey who beats up eight different deities in chapter four alone (4.36-4.45) to someone who says "Master [Wong Lao-Tsai], let me help you to your feet" (7.98).
Because of his journey, he's the perfect mentor for Jin. In fact, he basically ushers Jin through the same kind of transformation. Jin goes from self-absorbed, selfish, angry teen—most evident when he says to Wei-Chen, "'Maybe I just don't think you're right for [Suzy], all right? Maybe I don't think you're worthy of her. Maybe I think she can do better than an F.O.B. like you'" (8.123)—to a humbled, reflective guy who's willing to wait a month at a Chinese bakery/café in order to see Wei-Chen and apologize to him (9.103, 9.117).
How does Monkey get Jin to become a better guy? Monkey guides Jin by first transforming into Chin-Kee and terrorizing Jin's fantasy self, Danny, ultimately compelling Jin to be comfortable with his Chinese self. For more on this hop on over to the "Characters" section.
You know what they say: Change is the only constant. Let that be your major takeaway when you see Monkey.