What this interaction reveals about Brutus' character is that his naiveté and idealism prevent him from seeing when he’s being manipulated. He wanted to save the Republic from Caesar, and that was his only goal in life - apart from that, he could see nothing else and think about no other cause. This is why it was so easy for Portia to trick him into telling her everything. However, it didn't do her any good - she still committed suicide in the end.