The Character I want to describe is neither the Protagonist nor the antagonist. He's a side character. Yet, he's going to have a very big part in my story. He is a not so open personality with qualities like hatred, indifference, arrogance, suspicious behaviour .. Etc. But he's good from the inside and never likes to reveal his true self. My question is how do you describe him so that the readers will never be able to understand him and his personality is slowly revealed as the story progresses.
I'd recommend, as I frequently do, what you could call either the Hemingway "Iceberg" method or the "Sturgeon/Delany" method. Either way, what this means is that you should do far more world-building than you actually use in the story, and then only reveal the pieces that naturally enter the story, and only as they would naturally occur.
In this case, spend time planning out your character's full personality and back story. But don't worry about the audience ever learning it. Instead, you can use it to inform your character's decisions and dialogue at the points where he appears within the narrative.
In theory this gives you a rich, three-dimensional character, but without boring the reader with tangential details and unrelated backstory. Since I've been using this method, I've found that it gives me interesting details to draw from when I need them, and gives the story more of a feeling of spontaneity and life.