In Final Fantasy XIII-2 for the PS3 and Xbox 360, you can add the famous Odin eidolon to your party near the end of the game.
In order to add Odin as a party member, you’ll need to defeat him twice in two different time lines.
The first encounter with Odin is in Vile Peaks AF010. You access this time line from Archylte Steppe. Do all side quests in Archylte Steppe to open up the warp gate to Vile Peaks AF010. This primarily includes finding all of the hidden cactuar statues in Archylte Steppe. Once you access Vile Peaks AF010, do all of the side quests there and they should lead you to the battle with Odin.
The second battle with Odin is in the Vile Peaks AF200 time line. You gain access to this time line from Academia AF4XX. The warp gate should be near one of Chocalina’s stalls. Do everything possible in Vile Peaks AF200 to lead up to the second encounter with Odin, and once you defeat him here he will join your party for good.