Spoilers below ⬇️
So, from what I remember, you have to be nice and welcoming to Yasha, making sure not to raise suspicions from him.
Agree to Sage's plan.
When the option to text Sage appears, click "No".
Then Yasha will soon appear in the apartment, press the option "Grab anything that could be used as a weapon."
Yasha will chase you, at the train station, click the option "Push him off the station platform.", and that way you will get ending 9.