I personally feel at least one or two characters should be unlockable as you progress. Maybe only in the last level, but as I noticed people with bought characters seem to DESTROY me in the multiplayer because of spells, etc…
Zero Sum and Ace of Coders (the two highest difficulty tournaments) have 2 identical characters fight against each other to take away the advantage of equipment. You should be able to access them with a free account.
You could also try Harrowlands… most of the top 10 are using the starting heroes.
Otherwise, you could get a subscription.
But shouldn’t at least ONE character be free to unlock?