To use them, you must first purchase a Familiar-Gro Terrarium from the Market in Seaside Town.

The Terrarium can store one of each type of familiar, but you can only travel with one familiar at a time.

Familiars gain experience as you fight monsters. As they go up in level, they become more effective at doing whatever it is that they do.

To change which familiar you're currently using, or to rename your familiars, either click on the familiar's picture in your character pane, or go to your campsite and click on the terrarium.