Question: This site is exactly what I needed to actually pull out those "someday" ideas off the shelf, but I can't seem to find anything on character betrayals. It is the main thing holding me back from completing the plot and moving onto the first draft.
My main character has a brother who is the true antagonist of the plot, but plays the "good guy" role for a good portion of the story, playing along with the good-guy group until about the third act.
The best example that I can think of at this moment in time is Tohru Adachi from the game Persona 4.
How would I build up towards the brother's betrayal (reveal, perhaps?) without it seeming obvious it was going to happen, or choppy to the point that it happens out of nowhere and makes no sense? And how do I surprise the reader with this?
Often it's a case where a series of events is unfolding (such as the brother's motivation, what's going on in his head, or the actions the brother is taking), but the writer deliberately conceals all this from the reader. Maybe the reader only sees the results of the brother's actions, so they have a sense that something is going on, but they don't know why or who is behind it.
Much like a mystery, however, the story only really makes sense once the hidden events have been revealed (usually in the fourth act).
You will need to have clues planted throughout the story. Once the reader reaches the end, they should be able to look back and realize that the evidence was there all along, if only they had paid closer attention. The revelation of the hidden events should make sense of what did not make sense before.
And, of course, you will throw up red herrings or alternative explanations to stop the reader from guessing the truth too soon.
If you are a plotter by nature, you might work out the hidden series of events at the same time you write your outline, noting what will not be revealed until later.
If you are more of a pantser, you may not do such detailed planning. However, when you finish a draft, you will go back and rewrite the earlier chapters to plant your clues, now that you know what's coming.
Either way, you are correct in thinking that the betrayal cannot come out of nowhere. The clues must be there all along. And the brother needs a plausible reason for his betrayal. The reader needs to see in hindsight that the villain's arc unfolds in a way that makes sense.
Best of luck.