The last time we saw Bastion outside of Overwatch was in a short film showing his gentler side. That video, however, never revealed how he became part of the Overwatch team. Blizzard is wrapping up that loose end with the release of a new web comic called 'Binary'. In it, Torbjorn is sent out into the forest to hunt down the feared E-54 unit but comes away with a new ally.
You can read Binary on the Overwatch comic site or watch the MadeFire motion book.
