Pets
Pets hatch from eggs, level to 60, and act on their own timer during a fight - an extra on top of your turn, never instead of it. Rarer pets act more often: every 3 rounds for a legendary, every 5 for a common. Only your equipped pet uses its combat skill; the rest of the collection contributes stats.
The five elements
Which element
Damage elements suit builds that are already winning and want to win faster. Water and light suit builds that lose to attrition. Earth is the outlier: stun is worth more the longer a fight runs, which makes it the best pet for exactly the fights you are not sure about.
The skill tree
Every pet earns one skill point per level, 59 over its life, and a fully maxed tree costs exactly that - so max level means max tree, with nothing spare and nothing wasted. Tier 4 and tier 7 each force a choice between three mutually exclusive branches, so two maxed pets of the same species can still be built differently.
Star rank
Hatching a duplicate raises a pet's star rank, up to 5. Each star multiplies its combat output, which is what stops a second copy of a legendary from being worthless.