He's a tricky one, old Kichijiro.
On the one hand, this dude proves himself to be utterly unreliable and untrustworthy from day one. On the other hand, there's something about him that's deeply sympathetic, especially when you really put yourself in his shoes.
For their part, the priests distrust Kichijiro immediately. Well, okay, we can't blame them—Kichijiro is a shifty old drunk with the worst work ethic we've ever seen. Bad combo, right? What's more, Kichijiro "suddenly shut up like a clam" whenever the priests mention Christianity, looking as "unhappy" as a turkey on Thanksgiving (1.25). This makes the priests wonder if Kichijiro had known some Christians who were executed, though in their worst moments they suspect that the old drunk might be planning to betray them…
As it turns out, Kichijiro was once a Christian, but he renounced his faith to escape execution. That's why he's so nervous around the priests yet so committed to helping their cause. Basically, Kichijiro wants to atone for his sins. He may have renounced his faith officially, but that doesn't mean he's renounced it in his heart. Kichijiro's efforts work—for a while. He gains mega respect from the Japanese Christian community for bringing the priests, and he revels in his minor celebrity status.
As usual, however, Kichijiro trips up when things get hairy. First, he renounces his faith for a second time, not only stomping on an image of Jesus but spitting on it as well. Later, he reemerges and leads Father Rodrigues into an ambush, selling out the priest for "a number of tiny silver coins" (4.192). The dude has pretty much become the physical manifestation of Judas.
Rodrigues is understandably bitter about this turn of events. To justify his anger, he convinces himself that Jesus hated Judas, his betrayer, despite the fact that Jesus's whole deal is that he loves everyone. Shouldn't a priest know better than that? Meanwhile, Kichijiro continues to follow Rodrigues, desperate to alleviate his shame but utterly incapable of doing so. He even admits this weakness, saying that "one who is weak at heart cannot die a martyr" (8.58).
It's only when he grows to understand Kichijiro—and his similarity to him—that Rodrigues is able to forgive Kichijiro. If the poor guy had been born in any other place or any other time, he'd just be a regular dude—okay, maybe kind of a kooky dude—who goes to church a lot. But that's not the hand he was dealt. By showing empathy toward Kichijiro for the first time, Rodrigues is finally able to love his betrayer as much as Jesus loved Judas.