Pokemon Unite will eventually allow players to unlock up to 20 different Pokemon, but players won't start with them all. In order to get more, the player must first purchase a Unite License with either Aeos Coins or Aeos Gems. Players are also given one Pokemon for free, but this is from a limited list of only five of the 20 options. Still, choosing this first Pokemon wisely is important, as players will be stuck with them until they manage to purchase more Unite Licenses. There is also one Pokemon that is unique from all the rest in regards to how it is unlocked.
In most Pokemon games, players have to catch Pokemon in order to get them on their team, but that isn't how Pokemon Unite works. More akin to MOBA games like SMITE or League of Legends, players will only start with one of the 20 Pokemon unlocked in this game. One limited time Pokemon can be unlocked right away, and the other 18 must be purchased.
Once players start the game, they are given a choice between Pikachu, Charizard, Snorlax, Eldegloss, and Talonflame. One of these Pokemon can be obtained for free as a starter Pokemon. Next, players that log in to Pokemon Unite before August 31st will get Zeraora for free. This is a powerful mythical Pokemon that falls under the Speedster category of Pokemon Unite. Finally, the other 18 Pokemon on the roster are purchased with Aeos Coins or Aeos Gems.
It's best to think of Aeos Coins as the free to play currency and Aeos Gems as the paid currency. Both can be obtained simply by playing the game, but Aeos Coins will be earned much faster than the Gems, as these primarily come from events and other infrequent reward sources. All of the 19 remaining Pokemon (not including Zeraora) should be purchasable with either of these currencies, so Pokemon Unite can be played completely free. That said, players that are willing to spend money on it will be able to unlock more Pokemon faster than others.
It will likely take a while to unlock all the Pokemon for players that want to remain completely free to play, but as long as those players are having fun while they grind up the Aeos Coins, that isn't necessarily a bad thing. By participating in events, players will also be able to earn some Aeos Gems, so hopefully it won't take too long if players use both currencies to buy new characters.
Pokemon Unite releases on July 21st for Switch and later for mobile.