Cassandra Cain is the best person in the world who can read the body language of the people around her. After all, it was one of the first things her father David Cain made sure she learned to perfection from Cass's tender age.
As a result Cass can predict the moves of her opponents and anticipate them but not only that, she is also able to recognize a person's emotional state based on their body language and she knows that she is the best in the world in this field. And she often, despite the fact that she was forced to learn that skill, is still proud of herself for the fact that no one can fool her.
No one except Dick Grayson though.
At first Cass did not even notice that the emotions she was reading in her older brother were actually wrong compared to the real emotions Dick was experiencing.
Only later did Cassandra discover to her enormous surprise and shock that Dick Grayson is the only person in the world capable of hiding his feelings even from her.
This amazed her as much as it worried her. Because if Dick was able to deceive even her that meant that if Dick really wanted to, no one in the world would be able to know what he was actually feeling.
Cass once asked Bruce, the person who understands Dick more than anyone else in the world, if he was able to understand the emotional state Dick was in.
Bruce admitted that many times even he could not tell if his eldest son was really happy or if he was masking his sadness in the form of smiles and cheerful jokes or if he was constantly angry.
This only increased Cass's concern.
Soon she found out from Bruce that Dick is so good at masking his emotions and being a real actor because of his past in the circus where he was constantly performing with a smile on his face and managing to entertain the crowd of spectators and increase their enthusiasm.
Plus he too is one of the best in the world at reading the language of the people around him and anticipating their moves, just like her.
Basically, Cassandra does her best to figure out if her older brother is really happy or if he is hiding his emotions in favor of the people around him.
Years pass, but Cassandra Cain has never been able to solve the mystery surrounding the figure of Dick Grayson.