You will receive familiars as you progress through the story but once Esther joins your party you will be able to go out into the wide world and snare a few of your own. Unfortunately just defeating them in battle isn’t enough to secure a catch; it’s all down to chance as to whether they will take a shine to you and that level of chance decreases the rarer the creature is. So if you have your heart set on capturing a particular beast, be prepared to put in a lot of leg work and be patient!
Each person in your party can carry 3 familiars at any one time. If a carried familiar isn’t used in battle, it’ll still receive XP so it would be wise to pop a weak familiar in the 3rd slot that will enjoy the benefits of all that delicious XP without having to enter the fray and get you all KO’d.
A good mixture of familiars is crucial if you want to be prepared for any situation. Keep your bases covered with this line-up.
Fighters: Generally you’ll want to make sure that you have a good fighter with high stats across the board, although the Mag. Atk .stat isn’t that important.
Elemental: A familiar versed in elemental tricks with high Mag. Atk., Mag. Def., and Def stats.
Tank: Someone to soak up all the excess damage in the form of a tank, who will need high Def., Mag. Def. and Evasion. Any resistances they have will also be a handy bonus – a resistance to physical damage is particularly useful so look toward the automata and mortui genuses for candidates.