You just saw a new character and you don’t quite understand who he is. Let’s talk you and uncle about the Martian detective. In Zack Snyder’s Universe he had been with us since The Man of Steel . We just didn’t know.
Yes, the Secretary of Defense (first he was general) Calvin Swanwick (Harry Lennix), the character we had seen in Superman: Man of Steel and Batman vs. Superman, is, in reality, the Martian Manhunter, a hero who has spent years on Earth contemplating how heroes disagree and trying to do his bit to stop minor threats to our planet. By minors I mean minors than Darkseid.
The truth is that the character’s intentions are already quite clear to Bruce Wayne when he stands on his deck-dock. “I’m here to help,” says Martian Manhunter. “I have had many names and I have had many forms, and, like you, I have realized that I have a role in this world, and it is time that I start fighting for it,” he says.
It is the Martian Manhunter who poses as Martha Kent to help Lois Lane realize that she has to get on with her life without knowing that the heroes were trying to bring Superman back to life. Martian Manhunter is almost as powerful a character as Superman.
In addition to super-strength, invulnerability, and the ability to fly (which we have already realized that you have noticed that he moves the same when he flies as Superman), he is able to metamorphose his appearance and is a telepath. Almost certainly he is the one who from the future sends the visions to Bruce Wayne. This character is one of the original members of the Justice League, so it was inevitable that he was about to appear in the closest thing to a DC Cinematic Universe.
If you have seen or are a fan of Smallville, forget about the Martian Manhunter who appears in the series, put on Earth by Superman’s father to watch over him and protect him if necessary. It has nothing to do with it. In this case we are not clear how it came, but we do distrust the power of Superman. It is clear to us in Superman: The Man of Steel, especially now that we already know who he is. His interest in controlling Superman has to do with his ability to assess the dimension that Superman’s power represents. It is true that the more you know him, the more you trust him and his death, as we see in Batman vs. SupermanIt leaves him visibly affected, not to mention his misgivings during the fight with Doomsday (he remembers that he did not want to launch the missile at Kal-El). The message of the future from Flash to Bruce Wayne in that same movie has a lot to do with the Martian Manhunter’s misgivings: you were right to mistrust him. But not because Superman is a villain, but because he is a weapon of mass destruction that if it falls into the wrong hands is lethal to Humanity.