BINARY DOMAIN

This achievement is entitled: 'Rachael: Maximum Trust'.

Once the blue bar to the right of a character's profile in the start menu reaches the top, you'll get this achievement. To save time while thinking about conversation options, just stick around in an infinite enemy spawn zone, like the ones found near the end of almost every mission.

I did it when you get to a lift after rescuing two squadmates from the rising water in the Sewer mission You'll see little monkey bots start swarming you, so don't get on the lift just yet. Start putting rounds down into the bots. Making kills fast, getting multikills, getting headshot, etc, will increase their trust in you by impressing them. You'll see the icon for increased trust come up, and you'll hear them say something positive about you. Just keep it up, and you'll get them to full trust. It took me about 20 minutes, but I got both Rachael and Big Bo to full trust on Easy on my first run (didn't restart chapter or select it from Chapter Select).

However, later in the game (such as after you destroy the Amada corporation bots in the Resistance HQ) staying behind will result in trust being taken away. I don't know why this happens, especially when the bots there are just as easy to take down as the monkey bots and aren't too far out of your way. Maybe I crossed a checkpoint line or enemy spawner line that caused my turning back to be seen as desertion? If anyone wants to confirm or deny this, that would be great.