First of all, we meet the croaking frog (level 20) on Route 117.
Secondly, you need the Riptide trait! The Riptide Bubble Frog that evolves from 16 to level 36 and evolves into a Ninja Frog has a 50% chance of evolving into a Ninja Frog with the bond trait.
Secondly, you will have an NPC near the battle dome in the battle development area. Defeat him to get a characteristic capsule, which can be changed into a ninja frog. The ninja frog also has another characteristic that can be changed freely.
