https://wccftech.com/marvels-avengers-features-six-heroes-at-launch-story-choices-and-multiple-endings-confirmed/

Marvel's Avengers will have six playable heroes at launch, as confirmed by Scot Amos (Head of Studio at Crystal Dynamics) in an interview conducted with IGN Italy at the recent Lucca Comics and Games 2019 convention.

The Hero Missions are reserved for the single-player and put the player in front of levels built for the use of a particular hero and enhance the characteristics. Going forward you will also unlock the Warzone missions, playable with any hero and at any time, both in single-player and cooperative mode (up to four players).