Reaper was a Specialist in Black Ops 3 and with the latest update for Black Ops 4 Blackout players will get a chance to revisit the robot character. Unlike other Specialists in Blackout Mode, unlocking Reaper can only be done through purchasing the Black Ops Pass.
This is one of the characters that is exclusive to those purchased Call of Duty’s experimental season pass offering. The robotic character does have some unique sounds that he makes and definitely has a unique look compared to the other Specialists in the mode.
That said, anyone who has the Black Ops Pass will get Reaper and he can be found under the Multiplayer characters tab. There is no secret item and mission to complete for this character it can only be unlocked through the pass.
It’s not clear whether Reaper will come as an unlockable character at a later date, but there is currently no mission tied to this character like the other skins in Blackout. Perhaps there will be a mission at a later time so everyone can unlock the character.