Think of the most arrogant, egotistical person you know—that's the Monkey King. He's the character that no one likes. At first, that is.
At first, Monkey thinks he knows everything. And it is true—he knows an awful lot about kung-fu and being immortal. Some of his major kung fu skills: "Fist-Like-Lightning" (1.14), "Thunderous Foot" (1.15), "Cloud-As-Steed" (1.20), "Invulnerability to Wounds" (4.8), "Giant Form" (4.9)… it the list goes on.
In other words, he's super-powerful, and he's not afraid to show it. When he tries to crash the party in the heavens, he yells at the guard:
I, too, am a deity! I am a committed disciple of the arts of kung-fu and I have mastered the four major heavenly disciplines, prerequisites to immortality! (1.33)
But all those words don't matter when you're a monkey in a world of gods and goddesses who don't want you at their party.
Which is why Monkey goes around beating everyone up for not letting him attend the party in heaven and why he renames himself the Great Sage Equal of Heaven (4.16). (Monkey's not exactly into catchy names unfortunately.) It's also why he does things like command all the monkeys to wear shoes because all the gods and goddesses wear shoes (4.1), even though—clearly—monkeys don't need shoes.
In a nutshell? Monkey has a raging inferiority complex.
He's also remarkably stubborn and stuck on himself, even when he's faced with Tze-Yo-Tzuh, the creator of the Earth. Hence, even after Tze-Yo-Tzuh shows Monkey how truly omnipotent Tze-Yo-Tzuh is, Monkey goes straight into denial and says,
I don't care who you say you are, old man. I can still take you. (4.87)
If you're thinking Monkey's kind of impossible to deal with, you're not alone. No wonder Tze-Yo-Tzuh thinks the most fitting lesson for Monkey is to be buried under something as stubborn and hard as he is: a mountain made of rock.
When Wong Lai-Tsao enters the picture, Monkey's still the impossible jerk he was before. But Wong Lai-Tsao is a model monk, and nothing seems to fluster him. For example, when Monkey tells Wong Lai-Tsao that he has demons behind him, Wong Lai-Tsao responds thusly: "'Yes, I am aware of them. That is why I ask you to free yourself quickly'" (7.54).
Could you be so calm knowing demons were about to chomp you to pieces? We know we wouldn't be, and neither would Monkey.
Moreover, Wong Lai-Tsao is willing to accept his death if Monkey doesn't free himself and save him in time:
"If it is the will of Tze-Yo-Tzuh for me to die for your stubbornness, then I accept." (7.56)
How could Monkey not change himself because of Wong Lai-Tsao? The monk is willing to die for him, and he even lets himself get stabbed and placed on the roasting pit.
By the way, have you noticed that there's a monk in Monkey? We're pretty sure that's not by accident (and even if it is, it's a pretty cool accident). That's kind of the lesson for Monkey—to find his inner peace, his inner monk if you will.
Plus Wong Lai-Tsao reminds Monkey of a really good point: it's not really Monkey's true nature to be a monkey trapped under a mountain for eternity (7.66). In fact, Wong Lai-Tsao teaches Monkey to reclaim his own nature—to be a monkey.
Which Monkey symbolically does by kicking off his shoes when Wong Lai-Tsao tells him, "'On this journey… we have no need… for shoes'" (7.94).
So what does a monkey do after he reaches enlightenment? Become an Obi-Wan Kenobi guide to a confused Chinese American junior high kid. Obviously.
Sure Monkey's methods as a mentor/sage aren't exactly Zen—appearing as Danny's/Jin's worst fear, Chin-Kee, a walking Chinese stereotype run amok, and then fighting Danny/Jin in an epic kung-fu battle (9.6-9.41) is like the opposite of what the real Obi-Wan Kenobi did to guide young Luke Skywalker.
But we're also talking about the same Monkey who, while fighting one of Wong Lao-Tsai's demons, farts—complete with a charming "ppft!" and noxious green cloud emanating from his butt (7.83)—into the demon's face as a fighting tactic. Monkey's not afraid to get dirty—literally.
And why should he? Just because he's attained enlightenment doesn't mean he stops being a monkey, and last we checked, monkeys were pretty cool with all things dirty.
Another way to view Monkey's unorthodox sageness? He's just applying what he's learned from Wong Lao-Tsai—to be true to his monkey self, but in the service of the good (for example, serving Tze-Yo-Tzuh).
So Monkey uses his kung-fu and trickster ways to "'serve as [Jin's] conscience—as a signpost to [his] soul'" (9.71). And what does that mean exactly? Monkey never spells it out exactly, but we're guessing Jin's got the right message from Monkey when he goes to the Chinese bakery/café Monkey leads him to (via a business card from the heavens) and eventually meets with and apologizes to Wei-Chen.
But don't think that a business card is all that Monkey leaves Jin. He does have a final Obi-Wan Kenobi, deep-thought moment with Jin when he tells Jin:
"You know, Jin, I would have saved myself from five hundred years' imprisonment beneath a mountain of rock had I only realized how good it is to be a monkey." (9.82)
See? Monkey does have it in him to be a sage.