Each Familiar has two skills; one Active and one Passive. An Active Skill is automatically activated by the familiar during combat when the familiar is set as the captain. Since you can only set one Familiar Captain, choose your best familiar or the one with the most beneficial active skill to complement your character.
The effects of the Familiar’s Passive Skill will be available as long as the familiar is deployed. The passive effects of all deployed Familiars will be immediately available so if you have five deployed familiars, you’ll have five individual passive effects.