WEAR YOUR HEART ON YOUR SLEEVE
People who wear their heart on their sleeve express their emotions freely and openly, for all to see. Example: "She is kind of player who never hides how she's feeling. One look at her and you know if she's winning or losing." Reply: "She wears her heart on her sleeve."
You can see how these people feel as easily as if they were "wearing" their "heart" on the "sleeve" of their shirt. Example: "Some people love him and some people hate him, but my brother always lets you know how he feels. He wears his heart on his sleeve."
People who wear their heart on their sleeve do not hold back their emotions, for good or for bad. It is clear how they feel in each moment. Example: "She's a shy person. She's never been one to wear her heart on her sleeve."