She can be found near the Dilapidated Temple but in the last part of the game, to unlock Emma in Kuros Room defeat the main boss at the top of Ashina Castle then speak with the divine heir exhausting his dialogue, from there follow the smoke signals to talk to Ishhin, this will unlock Emma in Kuros Room. If you’re going for a different ending, without giving out many spoilers, I suggest looking at the purification route ending from the Sekiro alternate ending guide for Emma’s Locations.