I hate to rain on the parade of Tolkein purists, but since everything about the Second Age in the books reads more like a history or myth book, there's a ton of blank spaces to be filled in. That means that much of the cast is probably going to be made up of original characters not found in Tolkein's books who have stories of their own, which juxtapose with what is found in his books.
For example, there could be a soldier of the The King's men who while he believes that Men are better off independently, distrusts Sauron. And is also lucky enough to be on guard duty at the docks while Ar Pharazon is sailing to the undying lands. Thus saving him from Numenor's destruction.
Or a female Eregian Elf who suspsects something is out of place when Sauron offers to help Celebrimbor make the rings. And it's thanks to her, that the Elven Lords learn of Sauron's lies, and thus save themselves from his corruption.
Regardless, those who survive the early seasons and to the series finale should be in the Last Alliance, where the tension won't be who will win or lose, but who will live and who will die. If I was the writer, I'd call these orginal characters "The Forgotten Heroes", because their stories were never written, and thus, history forgot they even existed... until now.