Was confused for a sec because it said 4 great priests but there were 5 people, re-read and Lee is the high priest, when the other 4 are below him.
I presume is because his loyalty and ability to manage people is that high, even if is probably the one MC trust less, still a lot, but less than the other 4.
Really hope they can cure Osiris, I expected the damage to be reverted when they returned from the trial field, but nope, and I presume the slimes couldn't heal it when it was that extensive and old.