Chinatown Detective Agency

Once you have completed the first 3 missions as described in the other guide you will return to your office.

You will be told about paying rent, saving your game etc and be told you can now choose to work exclusively for one of the 3 clients.

Choose Keeran to unlock the achievement and then immediately press start and reloaded the checkpoint to do the same for the other 2 people (if you want to unlock all 3 of these achievements right away)

For me this characters option had disappeared when I reloaded the checkpoint so I quit out the game and fully shut it down. When I then started it back up and loaded back into the game his option reappeared 👍