Yllse doesn't know what to say after hearing what Xandre just said. She was so shocked that she was stunned in her position. She couldn't believe that he just confessed his feelings towards her. It was as if she was dreaming.
However, that's not what Xandre can see. All he sees was her expressionless face and it makes him feel broken. Yllse's lack of expression makes him misunderstand her feelings which is why he turned around.
He mumbled, "I knew this would happen." Xandre admired Yllse since they were training together. She was stunning and talented. But then, he felt like she wouldn't reciprocate those feelings which is why he tries to look for reasons to hate her. He hoped that hating her would make him forget about her romantic feelings.