[Game] I cannot register the material for the Pet Fusion.

When you Fusion pets, two or more Pets possessed can be added as material.
For Fusion, you need 4 pets of the same grade, and when the Fusion is successful, one pet will be obtained with one grade higher than the material that you have used for Fusion.
In case of Fusion failure, one pet of the same grade as the unit used as the material will be obtained, and the type of pet you can obtain by Fusion success/failure is random.