In-game Overwatch interactions aren't "strictly canon," lead writer says
Overwatch heroes Ana and Soldier: 76 are the only current team members who know that Reaper was once Gabriel Reyes—an original member of the Overwatch roster.
Though it may seem like more Overwatch heroes know Reaper's true identity because of their in-game interactions, Overwatch lead writer Michael Chu has reminded us on the Overwatch forum that in-game events "shouldn't be considered strictly canon." Interactions sparked when original Overwatch agents meet in-game—like with Reinhardt yelling "Traitor!" when he kills Reaper—are just there to add interest, Chu said.
Characters like Mercy that interact with Reaper in-game also don't necessarily know it's Reyes. And while it may seem confusing at first, it does actually make sense.
In-game Overwatch has no real bearing on the game's story—after all, players have all the real power in determining who is teamed with who in their matches. The Overwatch lore has nothing to do with two teams of six heroes repeatedly finding themselves in battle.
Instead, the story plays out outside the game in Blizzard's animated shorts and digital comics. Those are what we can squarely trust in regards to Overwatch lore. When Chu said that Ana and Soldier: 76 are the only two who know about Reyes as Reaper, he referred Overwatch fans to the Old Soldiers comic, where Ana and Soldier: 76 are reunited with each other and Reaper.
"Even with everything that's happened to Reyes, Ana and Jack would be able to recognize him immediately, given all their close experience serving together over the years," Chu said.