An open book
Definition 1 of 2
Pro Tip 1/3Predictable Personality
Refers to someone whose thoughts and actions are easy to guess. 
As soon as he heard the news, I knew he would offer to help; he's an open book.
Pro Tip 2/3No Secrets
Describes someone who doesn’t hide their feelings or secrets. 
You can ask her anything, she’s an open book.
Pro Tip 3/3Emotional Transparency
Indicates that a person's emotions are easy to read and understand. 
After the match, his disappointment was an open book.