Pets can be synthesized to level up. Put the pet into the Synthesis Slot, make sure Synth Stone is in the bag, and click “Synthesize” to upgrade. The synthesis chance gets lower as the pet level goes higher.
Pet skill increases with pet level. Every 3 levels up will grant you an opportunity to add another Pet Skill Slot.
Currently, the pet is available to be synthesized into +27 and can evolve forms in +9 and +18 phases.