the suppliant was very angry

The suppliant person humbly begged for forgiveness.

In this context, "suppliant" means someone who is making a humble or earnest plea. Kino putting on his "suppliant hat" suggests that he is preparing to ask for something in a respectful and pleading manner.

benefactor, supporter

Both "supple" and "suppliant" relate to flexibility or adaptability. However, "supple" typically refers to physical flexibility or elasticity, whereas "suppliant" often refers to being submissive or making a plea for something.