I think we already got a glimpse of Negan . I was watching the show with my wife who had never seen the show and in season 5 when the terminites are having their flash back to when they were locked up in the box car, the door opens up and you see a siloute of a man in a jacket. He has the same build and hair cut as Negan. It would make sense because the group at terminus was originally good until they were taken over.

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