A new hero, Ashe, along with her companion Bob, are joining the fight. Ashe is a leader of the Deadlock Gang. She was featured during the McCree animated short on BlizzCon 2018.
She is going to be a damage character, with her rifle, dynamite, and other weapons. Moreover, she has various abilities aimed to deliver the pain to her opponents. You can look her up on the official Overwatch website.
You can watch the reveal here (thanks to Gamespot for the video):