Garnet:[/b] Like Vanille, she has fantastic stats in both Magic (119) and Mind (112) (Vanille has 106 and 116). However, her Magic stat is only useful if you use summons (she has 5* Summon). Both have access to 4* White Magic, making them great healers. Her SB increases your whole party's attack by 20% (which is slightly less powerful than Boost, but it does affect the whole party after all). For a healer, she is slightly more sturdy than the others, thanks to her 3015 HP score and 55 Defense (and she has access to her memory crystal after all). Finally, she has some versatility, thanks to her 2* Support skill, though you may never equip her with such abilities.
[*]Vivi: The black mage par excellence. He is the best black magic caster you can unlock at this point, with 129 Magic points at level 50 (and he can reach 159 at level 60 thanks to his memory crystal). His regular Soul Break increases his magic, giving him a +50% boost (duration?). If you wish to deal tons of damages, Vivi is your man. However, his defensive stats are very low (nearly identical to Rydia, with 2613 HP and 53 Defense at level 50), and he can only equip Hats, Robes and Bracers. Because of that, and since it takes a while before leveling him to level 50 and beyond, Terra might be a better candidate since she is much more solid. Finally, he has absolutly no versatility, since he is only able to cast Black Magic (but that's all he needs to shine).
and I assume Dark attack?