Is this game free? Yes, play without ever spending a penny.
Is this game pay to win? No, many of the best decks in the game do not use very rare cards. Matches are largely determined by board positioning and skill as well; not which cards you have.
Are all cards playable forever or are there rotating sets? All cards are playable forever.
How does balancing work in this game? All cards are designed to have abilities and stats that can be incrementally adjusted over time. Between movements, abilities, HPA, cost, etc there are many different ways to moderate the power of every single card.
What's the best way to meet other Dark Duel players: On the discord linked here: Discord.