I'm not that far into the game (at least I think so). Right now I'm at the end of Palumpolum (like mid of chapter 7 I think) and so far I've got the roles leveled to level 2. But I really don't know which character should take what role as main. So far I tried to level all roles equally.
sorry i am too used to the japanese names for the roles fang - attacker (commando) snow - attacker (commando) vanille - jammer (saboteur) sazh - enhancer (synergist) hope - blaster (ravager) lightning - blaster (ravager)
defenders are the same as sentinels healers are the same as medics
Psalm 73:20: As a dream when one awaketh; so, O Lord, when thou awakest, thou shalt despise their image.
Fang has the highest physical attack in the game and should almost always be a Commando. If you plan to use Vanille as well, you should also build Fang's Sentinel role and leave her Saboteur role for last. Otherwise, build Fang's Saboteur role second as she is the second best Saboteur in the game.
Snow - Sentinel
Snow will never reach the attack power of either Fang or Lightning but what he does have is HP and a lot of it. Snow should be a Sentinel first and a Commando second. Don't even bother with his Ravager role unless you have nothing else to spend CP on.
Sazh - Ravager / Synergist
Sazh will never deal great damage but he makes for a great offensive buffer and support character. Spend his CP on his Synergist role first. The Ravager role is not about dealing damage but rather about raising the Chain gauge so he should spend his remaining CP in his Ravager role. The only time you want him to be a Commando is when he is alone with Vanille.
Lightning - Commando / Ravager
Lightning is probably the most balanced character in the game. She has an equally high physical and magical attack power and has abilities which cater towards both offense and towards support roles. You should build her Commando role with priority first since she will be the best Commando you have until you get Fang.
Hope - Ravager
Hope has the highest magic attack and makes a great Ravager. However, at the same time, you can easily make Hope a Commando later in the game when you gain access to all 6 roles for all characters. Commandos have both magical and physical attacks and their role bonus increases the damage for both so Hope will deal a lot of damage as a Commando.
Otherwise, he is the best defensive buffer in the game with the Syngergist role. Build up his Ravager role primarily followed by his Synergist role. Make sure to spend any extra CP on his Medic role if you plan to use him and not Vanille. Once you open up all 6 roles for everyone; if you are still using him, spend 6000 CP to obtain the ability Ruin for him in the Commando role. He will deal damage on an equal scale with Fang (at least with normal attacks).
Vanille - Saboteur
Vanille is probably the best Saboteur in the game but she is missing some notable Saboteur abilities that Fang obtains. Still, with the second highest magic attack, a weapon ability which increases the chances of weakening status effects, and the Saboteur spell Death; she is best suited as a Ravager / Saboteur.
She is also the only character who has a weapon ability which improves cure magic so she could be your best Medic as well. Otherwise Hope will be the best Medic by default since he has the highest magic stat and they both gain the same abilities.