Finding rare treasure requires someone with low Brave. The percentage chance you have of finding a treasure is inversely proportional to how much Brave you have, I believe the forumla is as such:

Ramza has 97 out of 100 Brave. Therefore, his chance of finding a rare item is 3%. Rafa has 35 Brave out of 100. Therefore, her chance of finding a rare item is 65%. 100-97=3, 100-35=65. Subtract a character's Brave from 100 to see their chance of finding a rare item.