[Game] How can I level up my pet?

Pet level can be raised through hunting, pet material above Rare, and Exploration.
Pets that can be used as materials are more than one Rare pets you have.
You can check the information about which pets are available for material at [Pet] → [Pet Level] → [Material Register Activate].
After the materials are chosen, tap on the [Pet Growth] button then you can level up your pet.