One Dog Story (UP)

Calvin is a brain in a jar that you will most likely stumble across during your adventure. I say 'most likely' because I don't think there is anything required in the room that is necessary to progress the story, so I suppose it is missible. Unfortunately, I don't remember exactly where the room is. However, I can tell you it is after the Cthulhu boss and before the Keeper boss.

But, you should be checking all of the rooms anyways, right?

Anyway, once you find this brain, it will ask you to kill it. For this achievement, agree. He will die, and you will unlock an achievement.

For the 'Restless Mind' achievement, simply pause the game after the achievement unlocks, select 'load game', find Calvin again, and choose the second option.

It should be noted that regardless if you kill Calvin or not, it will not effect any of the endings, so you can proceed regardless of which choice you end up with during your play.