A familiar will possess 1 of 8 celestial signs. In addition to the 4 basic signs of sun, moon, star and planet, there are special twin signs that are twice as powerful as their single counterparts. The planet and double planet signs are the rarest and most desirable of the signs due to the XP bonus they bestow on the familiar.
Sun – +5% resistance to fire-type attacks and sleep
Star – +5% resistance to storm-type attacks and poison
Moon – +5% resistance to water-type attacks and confusion
Planet - +5% XP bonus
Twin Sun – +10% resistance to fire-type attacks and sleep
Twin Star – +10% resistance to storm-type attacks and poison
Twin Moon – +10% resistance to water-type attacks and confusion
Twin Planet - +10% XP bonus
Extra bonuses arise in battle depending on the signs of the creatures in the field. With the exception of the planets, each sign has a weakness against one and a strength against the other similar to rock-paper-scissors; sun is stronger than moon; moon is stronger than star; star is stronger than sun. A familiar with the dominant sign will deliver 20% extra damage to the weaker sign, whilst a familiar with the weaker sign will deliver 20% less damage in its attack to the more dominant sign. The percentage stats rise to 30% when the attacker belongs to a twin sign.
The planet signs are much less convoluted. A familiar with a single planet sign is granted a 50% damage bonus against a familiar with a twin planet sign, whilst a familiar with a twin planet sign gets a 10% damage bonus against all sun, moon and star signs.
For the purposes of growing your familiars, the planet signs are the best. The star signs are also favourable as many of the monsters that crop up in bounty hunts fall under the sun sign so you’ll have the advantage of extra damage bonuses. Moon signs have a disadvantage against bounty targets so if pick up any bounty hunts in your travels, be sure to enlist the help of familiars with sun signs.