Bahamut
How to Obtain
You can get Bahamut in disc 3 when you have the Rangrok.  Fly to the farmost SW corner of the map.  There you will see the resarch lab.  Land on it.  When you enter the lab only walk towards the core when it isnt glowing.  When you reach the core you will be asked a question.  Pick "It is not our will to fight"  then you will fight a red dragon. Then you will be asked another question and answer "Never".  And then you will fight another red dragon.  At the end of that fight you will be asked yet another question to which you have to answer a secret option under the two top options.  There you will fight Bahamut and when you beat him he will join you.
Description
The view changes to the full moon and Bahamut flying through the clouds, then he dives under then and reaches the enemy.  He spreads his wings and gives a yell.  Then fires multiple non-elemental balls of energy at the enemy, and it all explodes.
Iggy's opinion
This is probably my favourate GF of all time.  Where else can you get a huge dragon to help you.  Well this is the other GF that has the mug ability which i use almost all the time.  And i just love it couse it looks realy cool as well and its my favourate since FF5.  9/10
Ability
Ability                   AP         Description
Ability x4
Magic
GF
Draw
Item
Mug
Move-HP Up
Auto-Protect
Expend x2-1
Rare Item
SumMag+10%
SumMag+20%
SumMag+30%
SumMag+40%
GFHP+10%
GFHP+20%
GFHP+30%
GFHP+40%
Boost
Forbid Mag-RF
Learned
Learned
Learned
Learned
Learned
200
200
250
250
250
40
70
140
200
40
70
140
200
10
200
Equip uo to 4 abilities
Use Magic command
Use GF command
Use Draw command
Use Item command
Use Mug command
Restore HP as you walk
Automatically cast Protect at the beging of battle
Cast 2 magic for the price of one when on double
Easier to get rare items
Raise SumMag by 10%
Raise SumMag by 20%
Raise SumMag by 30%
Raise SumMag by 40%
Raise GFHP by 10%
Raise GFHP by 20%
Raise GFHP by 30%
Raise GFHP by 40%
Boost GF
Refine Forbiden Magic from an item
1