You can find some information on the different Veil Demons currently available on the Breach wiki.
The developers have said they are against any kind of pay to win features, focusing their monetization on purchasing cosmetics, or classes which can be unlocked with an in game currency you earn as you play, in a similar manner to League of Legends or other MOBAs.
Breach's progression is horizontal rather than vertical, meaning that as you play you unlock more means of customization rather than power. Leveling up is tied to the missions that you play, again similar to MOBAs where you start at level one. You compete for experience against the Veil Demon as you work to complete objectives.
Gear and gems have already been confirmed to not be purchasable with real money. They are earned from completing missions (win or lose) and I believe by purchasing them with the in game currency only. Gear modifies the talents you have access to as you level up in a mission and gems give a small % boost to base stats