And here's a spoiler on Odin. If you want read it, it concerns an extra scene in the game and a new GF

(SPOILER)If you have gotten Odin BEFORE the battle with Seifer in Disk 3, he will appear in the beggining of the battle and Seifer will kill him. Then, continue the battle and DON'T beat Seifer. Take the chance to stock up Auras and heal you members, but DON'T attack. After a while Gilgamesh will appear and beat him for you. Gilgamesh is a new GF exactly like Odin. Can't be equipped, but appears randomly. She will have four different swords. Some just damage, others (like Zantetsuken) kill.

Oh, and raise up your luck as much as you can. The greater the stat, the more often Odin will appear.