Here’s a 200th level pet and how I think it works. Subject to change where I’m wrong.
See those tiny green arrows inside the boxes? They show that this pet affects the hero you place in the center of your back row and the hero just above it in the back row. (There are a total of 12 boxes which represents all possible positions of all the heroes in your team and your opponent’s team. Your hero placements are the 6 boxes on the left.)
The first thing this particular pet does is give both my heroes +537 magic defense. I could increase the effect by upgrading it to level 201 by clicking the upgrade button.
It also gives 3 additional effects for my two heroes. The first was granted at level 70, the next at level 120, and the next at level 200. I can see what they are in the game by clicking on the individual hexagons.
The next screen shows the mutations. Both of my two heroes also gain all the effects listed here. I like the 1st and 2nd mutations so I’ve locked them in using the padlocks. The more I lock, the higher the cost to mutate. For 90 crystals, I can currently mutate the last two mutations into two other mutations.
The next screen allows me to attempt to increase the pet star rating. I have collected 12 of the 90 required pet fragments. I need to collect 78 more. Once I do, I’ll be clicking on the rebirth button for a 14% chance to make Wrinkle a 6-star pet. If I fail, which is likely, the chance next time will be slightly higher. Repeat, repeat, repeat. Note that pet level will reset to 1 each time I make the attempt, whether I succeed or fail.
The good news is that I do not need to level a pet at all to make the attempt. I do level the pets I actually use on my teams at least a little bit, but I don’t go overboard and I definitely don’t level the ones I don’t use. It would be a complete waste of resources each time an unused pet attempts a rebirth.
And lastly, we have support skills.
If I have all 4 of the first set of pets on my team in addition to Wrinkle, then my two heroes will also each gain +12% physical defense.
If I have all 6 of those slimy pets on my team in addition to Wrinkle, then my two heroes will also each gain +12% magic defense.
Here’s where I think the fine print kicks in. If a support pet is 5-star, then I get credit for it even if it isn’t on my team. So, if Wrinkle is on my team and all 6 of the slime pets shown here are 5-star or higher, then my two heroes would gain +12% magic defense even if none of these slime pets are actually on my team.
And lastly, my Wrinkle provides +20% magic attack to two heroes in the back row. Guess where I put my two mages.
It’s not really a guide, but it is an example. Hope this helps!
Update: Blessed with a small miracle today when my 6-star Wrinkle rebirthed to a 7-star Wrinkle with just a 7% chance. Since 7-star is currently the max, I therefore fully upgraded it to level 560. Here it is, for those interested.