Questions about Hero wars avatars upgrades.

Hey everyone. Those who is know what the fuck is With pets, who is like power ups for heroes in Campaign mode. I was upgrading one pet in max level, and all rest pets is valuable "put on" Heroes. But no effect. Can someone explain, what do for max effect. Shoul i have upgrade all pets tom max level or that skill is only for one basic pet? Ps. sorry my english is not good. :)