Frown angrily and tell her that's impossible

"Don't be absurd!" said Machiko angrily. She pulled her hand away and rose from the bench, meaning to return to her chamber at once. "I am a royal princess, you are some sort of ninja-girl who thinks she is a man!"

The black-clad figure looked stricken, and for a moment Machiko thought she saw a wounded look in the stranger's eyes, but before she could react, the mysterious woman had gone as quickly and silently as she arrived.

A little shaken and upset, Machiko returned to her room.