Hidden Quest

As the ancients said, 'To do a good job, one must first sharpen one's tools.' Regarding hidden quests, you usually need to accumulate capture counts or achieve specific feats to raise your pet's rank. This is all for your own good, so I recommend focusing your resources on nurturing your core pets to break through bottlenecks!

If I understood your hidden quest correctly, you need to go to the pet gacha page, and there's a level in the top left corner. You need to get that level to 3. (I finished playing a while ago, so if the hidden quest has been changed, this might not be it. I'd be more sure if there was a picture.)

Keep on pulling