The Story Goes On

Foster can only be unlocked in the Spooky Forest level. While playing the Spooky Forest level, keep your eyes peeled for a set of footprints, seemingly being made by an invisible person. They'll show up in a random room. Follow them from room to room until they stop in the middle of a room at the top of the map. From here, go straight up into a new hallway, take it to the end, and break Foster free to unlock him and this achievement.