Diablos
Elemental: None
Starting Level: 9
Starting HP: 730
Where do I get it? Talk to Headmaster Cid before leaving for Timber and he'll give you a cursed magical lamp which gets added to your Items list. To use the lamp go to your menu and go to Items and look for the lamp. Then when you use it you'll get teleported into a battle against Diablos. You have to beat him to have him join your party so I'd level up a bit before using the lamp.

GF Attack: Dark Messenger: This attack seems really wussy at first but as you build up its level the amount of damage done gets bigger. For example if you're Diablos is level 9 then it does only 9% of your opponents damage. If its level is 50 then it does 50% of your opponents damage and so on. (I think you get the pattern by now) I bet you're thinking "whoa if i get i get my Diablos at level 100 then it can completely kill off the enemy." well thats not really how it works. the maximum amount of damage diablos can deal is 9999 and sometimes enemies are immune to his attack.

Useful Abilities: Diablos has 2 useful abilities: Mug and Abilityx3. Abilityx3 lets whoever is equipped with Diablos equip three GF abilities instead of the usual two. Mug lets you steal from an enemy and attack at the same time. When you mug enemies you could be getting useful rare items for refining weapons and other things.
Ability List
Ability

HP-J
Mag-J
Hit-J
Abilityx3
Magic
GF
Draw
Item
Dark
HP+20%
HP+40%
HP+80%
Mag+20%
Mag+40%
Mug
Enc-Half
Enc-None
GFHP+10%
GFHP+20%
GFHP+30%
Time Mag-RF
ST Mag-RF
AP Needed

50
Learned
120
Learned
Learned
Learned
Learned
Learned
100
60
120
240
60
120
200
30
100
40
70
140
30
30
Leads To...

HP+20%
Nothing
Nothing
Nothing
Nothing
Nothing
Nothing
Nothing
Nothing
HP+40%
HP+80%
Nothing
Mag+40%
Nothing
Nothing
Enc-None
Nothing
GFHP+20%
GFHP+30%
Nothing
Nothing
Nothing