A soldier from the future who has come back to kill a child, for reasons that are entirely sound if you have seen what the child becomes.
Cable is the straight man the second Deadpool film needed. Josh Brolin plays him with no jokes at all, which is why the film can keep making them. He has come back from a future where a boy called Russell burns everything, and he intends to prevent it in the simplest available way.
The film’s decision to give him a photograph, and to make the whole thing about a family he cannot get back, is the reason the character is not just a mechanism.