Echo was announced to be the new hero for Overwatch but it looks like the character was actually sketched for another game that did not make it out.
According to Blizzard, Echo was supposed to be a character in Titan which is an early project that got canceled before Overwatch 1. It was explained that Titan was an MMO project but they later choose to kill it off and a lot of what they created during the development period was alter repurposed into Overwatch.
He added that Echo was one of the characters that they did like from that time and decided to bring it to Overwatch. They decided that the Echo would be a part of Overwatch right from the start but had to figure out how she will fit into the universe and also figure out her abilities and role.
Blizzard also added that Echo will be the last hero for Overwatch until Overwatch 2 arrives.