Well you have to get to the top of his tower first. See, you're on a time limit, and there are going to be some puzzles you need to do. They're a bit of a *****, look up a walkthrough. THe more annoying thing is though, you're constantly going to run into Toberries and Skeletons, and those guys are bitches. Just run away from everything, you can come back to them later. Now, when you do get up there, Odin will actually just stand there, and if you pummel him into submission within the time limit, he'll join you. He uses an attack called "Zantesuken" which insta-deaths every enemy on screen, but he'll only show up every once in a while. If you do it before the third Seifer fight though, Seifer kills him, and Gilgamesh replaces him. Gilgamesh uses three other swords, not all of them kill though, but there is a chance of him showing up in boss fights. Once you've beaten Odin's tower, you can come back and murder 20 tonberries in a row or so, and you can kill the Tonberyy King, who will become a standard GF, and he's got some decent junctioning and abilities to use.