All of the game’s 300-plus familiars are divided into 14 different genuses. While there are differences from familiar to familiar, members of the same genus typically share the same strengths and resistances.
Each genus also excels at certain abilities and is likely to gain points in those abilities more quickly than in other abilities. Each of your four human party members specializes in dealing with certain genuses, and your familiars will receive a 10 percent bonus to their abilities when they’re being used by that particular human.
(When moving a familiar from one character to another, look to the area above and to the right of that character’s roster to see the icons showing which familiars he or she is most compatible with.)
While players should definitely take advantage of that bonus when they can, make sure each character has access to a good variety of familiars.
For example, Oliver may be most compatible with aggressive fighters, but it’s still to your advantage to make sure that he has access to an elemental trick performing familiar, even if there’s no compatibility bonus.