From what noticed… Earning their respect is the key in getting their romance.
Passing stat checks will earn you points if you go with them during the infiltration mission. Being pragmatic instead of being principled will net you points as well when you’re with them.
Based on my playthrough,
Summary
- On Chapter 1 you shouldn’t kiss them whe you flirted with them instead Back out…What you do on confronting the man and Petrovich could earn or lose some points depending on how you do as well.
- When you meet Angel again during the party, choose to shake hand with them warmly
-During the main infiltration mission, choose to go with them. Make sure to be pragmatic with your interaction with them and pass all the stat checks to impress them.
-When confronted whether to kill or capture Fautus, make sure to choose to injured but capture him.
-After this if you earn enough respect, it will trigger the start of the romance scene. Choose to let them have the suitcase and done.
Edit: The author just recently shared how to get the specific scene.