You'll encounter Cerberus in Galbadia Garden during the War of the Gardens event. Approach the boss and defeat it in battle and it'll join you. If you don't defeat Cerberus when you have the chance to do so on disc 2, you'll have to wait until you fight Gargantua on disc 4, whom you can draw Cerberus from. When summoned it'll use its Counter Rockets ability, which will cast Double or Triple on the entire party.
Cerberus Base Compatibility
Cerberus Compatibility Factors
|Compatibility Items||Dragon Fin|
|Favored Magic||Aura, Dispel, Double, Esuna, Haste, Protect, Reflect, Shell, Triple, Ultima|
|Opposed Magic||Berserk, Blind, Break, Confuse, Pain, Silence, Sleep, Slow, Stop|
Cerberus starts out with Str-J and the rare Hit-J abilities learned and can also acquire Mag-J, Spr-J and Spd-J, so as long as you keep it an Pandemona apart you shouldn't be squandering junctions. Cerberus is only one of two GFs to know the ST-Def-Jx4 ability, which will give you great protection against status effects, as is the third GF to come with Ability x3.
Other than those incredibly useful junctions and abilities, Cerberus also comes with Spd+20% and Spd+40%, the lovely Auto-Haste ability (grants the equipped character Haste), Expendx2-1 (only expend one spell when doublecasting spells) and Alert (prevents the party from being back attacked).