The mestizo, a word once used to describe people of mixed European and Native American ancestry, is a symbolic character that is not fully developed. Graham Greene developed the mestizo only so far as to fulfill his symbolic function so he remains a flat character who never develops and so is static. His symbolic function is that of representing the Judas figure in the whisky priest's life.
The encounters between the whiskey priest and the mestizo provide opportunities for the whiskey priest to attain or reclaim some higher purpose in his own call to the priesthood--he can step outside of his own barriers of guilt and inadequacy and do something for someone in need.
The mestizo, the recipient of the whisky priest's kindnesses, is belligerent and ungrateful and turns his back on the priest by tracking across the border. While he is thankless and heartless, he is greedy and asks the captured priest to pray for him. The mestizo thus symbolizes the betraying Judas figure in the story. Nonetheless, he is not wholly without sympathy because we see him through the priest's eyes while he ministers to the mestizo with kindness; we see him as the priest visualizes him:
When you visualized a man or woman carefully, you could always begin to feel pity .... When you saw the lines at the corners of the eyes, the shape of the mouth, .... Hate was just a failure of the imagination.