The Centra Ruins is located near Edea's House. You can It holds two Guardian Forces.

¤ Odin

When you first enter Centra Ruins there's a 20 minutes time limit running. To avoid random encounters equip Enc-None. Get into the tower by following the staircases and then get into the lift to go to the next floor. Then climb the right ladder and hit the blue ladder at the top and go back down. Hit the blue light to make a hidden staircase appear. Climb those stairs to the next level. You'll see a statue to the left, inspect it and remove the eye. Then head back and climb the ladder to the roof of the tower. Climb up to the top of the dome and place the eye you have just taken into the statue. You'll get a random code. Remember it and remove both eyes and go down to the statue and place both eyes. Then you can enter the code. Enter the code and if it's correct the door below will open. Enter it and you are in Odin's Chamber. To get the Odin GF you have to defeat him.

Odin isn't really a challange. Just hit him with your powerfull strikes and he be history soon. If you don't have Triple yet, be sure to draw it from him. After you defeated him he'll join you. Odin cannot be casted, but will automaticly be summoned randomly in a battle, but not in boss battles.

¤ Tonberry King

To get Tonberry King you have to re-enter the Centra Ruins and fight against 20 Tonberry's at the same place. Then the Tonberry King will appear. Cast Diablos a lot of times and he'll be dead soon. After he's defeated he will join you.