The Roster page, accessible from the home page, allows you to see all the heroes in the game, owned or not.
When sorted by Threat, heroes with enough shards to unlock are listed as if they are already unlocked and at minimum stats. Heroes without enough shards are listed at the end separately.
When sorted by Name, heroes are all sorted alphabetically regardless of unlock status, first by their character name (including changes), then by their variant name, with the "prime" version listed first (e.g. Black Canary is followed by Sonic Black Canary, but Multiverse White Canary is near the end under "W").