Warrior of light
(above all available with their champion medals)
He's at the end of the pier near the stairs to the crystal tower, near where the boy wants to see 10 different specific mirages. Get 10,000 points fishing and it should allow you to angle a really big fish, I just mashed all the buttons at once while this certain fish was hooked as there was no penalty in hitting the wrong button.
This will also net you the achievement for fishing up leviathan.
Both come from completing the intervention quests that come up post true ending
Y'shtola comes from collecting all the items from the white cats across various npc based areas
Firion I believe comes from another collection item one of them is in nibelheim that I, at least, couldn't collect until post true ending
Cecil comes from an intervention quest based on him