Pets are taken into the arena by players and provide them with one active, one triggered, and a choice of one of two passive bonuses.
You start with a pet of your choice from Pincer, Tortus, and Mystik. You then play games to earn seals or pay money to unlock more pets of your choice. Pets you unlock all start at lvl 1. Lvl 1 pets only have their activated ability. When the pets evolve, they will unlock their other 2 abilities (a triggered passive and a passive boost), and each level one of their existing abilities will improve.
Pets level as your account levels, meaning any new pets you unlock will be the same level as any other pets you currently have.
You can customize the look of your pet by purchasing skins with real money.
You can not use money to purchase levels on pets, though boosts will be offered. You will not have to have multiples of the same pet - while there are 2 choices for the passive boost, you'll be able to switch between them for a little commodity (not money).