To get this ending, you need to reply to Katherine’s messages about the pianist. Specifically you need to choose the options including “heard it”, “luck”, and offer to send a video.
You should get a response from her, you can send pretty much anything. Some guides say you need to get a response from her, I didn’t get one.
Stage 9.2 onwards I chose yes, yes and peaceful. Ending popped from me during a heavy chaos playful (half to three quarters across the bar).