In order to obtain this achievement, you must
win Jonas' sympathy during the game. To do this, try to be nice and comprehensive with him, and respect these very important points.
VERY IMPORTANT: This achievement won't unlock if you meet the requirements for "Thicker than water" at the same time. More info inside the spoiler tag.
Beware of spoilers.
*** Spoiler - click to reveal ***- In the few ocassiones when you are accused of causing the poltergeist, DON'T BLAME JONAS. Blame yourself if needed.
- Choose to take Jonas with you when you go find the Adler Estate keys.
- DON'T accept the ghosts' offer about Clarissa when you are on Bridge Stand attemting to open the bunker. This is specially important.
- Help Jonas with the tape recorder inside the bunker.
IMPORTANT: You need to convince Michael to leave town during the past scenes. It doesn't matter if he's still with Clarissa or they break up, but, in the last conversation with him at your house, tell him to leave. If you don't, you'll unlock "Thicker than water" instead of this achivement. Thanks tobiasvl for the confirmation.
If you meet these requirements, you'll obtain the achievement during the "stat count" before the credits.
For a guide on how to get all the ending achievements in the lowest number of playthroughs, click here