To the new cat owners out there: here is the progression for your kitties. Starting with level 2, the order is:
[list][*]Level 2: Another Active
[*]Level 3: Another Passive
[*]Level 4: Active Upgrade
[*]Level 5: Passive Upgrade[/list]
And the order repeats with additional levels. Each cat can have 4 actives and 2 passives maximum. As a reminder, your cat gets a level upgrade per encounter (these encounters don't have to be the formal ones that reward bonus xp). However, if there is more than one cat in the encounter, the one that gets the upgrade is determined via two methods: whichever cat is lowest level, or if all cats are the same level, random chance.

Special note for all Mewgenics cats: instead of dying, they will be downed with a random stat-lowering injury. At that point, if they take three hits from ANYTHING while downed, they'll be permanently killed. Revival abilities will revive them, while an instance of healing will add an extra hit. If a cat ends a battle downed, it will not be eligible for a level up.