Emma is a non playing character in the Sekiro game. She is located in the Dilapidated Temple and you can easily counter her with your sword.
Follow the steps below to kill her.
How to defeat Emma:
You can counter Emma with the use of floating passage within one minute. Just attack Emma with the floating passage and block her strikers with your sword.
After the block, hit her again with the floating passage.
She will become weaker and then strike her neck following the red spot for the final blow.