This hero is speculated to be known as Sombra and could end up being the mother to one of the heroes in the game right now, Pharah. How did they arrive at this conclusion? Basically this was started by a French cinema-chain who had initially uploaded a post claiming that a new hero would be unveiled, but they later backtracked saying that it was a typo.
Now just in case it was indeed a typo, there is other evidence to suggest that Sombra does exist and could be added to the game, such as files spotted in the game/cinematics with her name on it. There is also some backstory and history as Sombra is one of the original members of Overwatch before it was disbanded, and characters like Reaper (when he used to be good) and Soldier 76 were part of the same team.
Blizzard had also announced that there will be a new animated short this Monday starring Soldier 76, which is why it is believed that that could be when Sombra is introduced. So far Overwatch’s shorts have included two heroes everytime, like Winston and Reaper, Tracer and Widowmaker, and more recently Hanzo and Genji, so maybe we could get a Soldier 76 and Sombra short next. Either way do check back with us then for the details.