The Keeper

Lore & Background

McMillen had hidden secrets in the original game and in Rebirth, including the Lost, a playable character whose unlock needed a series of steps such as dying repeatedly under specific conditions. Players on the game's Reddit forum found the Lost by searching the executable files, and discovered it and its unlock within 109 hours of release, which disappointed McMillen. For Afterbirth he wanted to hide the Keeper, along with elements about Isaac's father that the game already hinted at, but knew players would data-mine the files again.

Instead he planned an ARG requiring real-world clues, arranged to run without him because his daughter was due at the end of September 2015, a month before the expansion's release. When Afterbirth shipped, players found what looked like bugs, such as promised items that were missing; some of the omissions were part of the ARG, but the build also lacked some items by accident. McMillen's team patched the game and offered hints about the Keeper using the number 109.

Players then found in-game clues and followed them into the ARG, which involved calling a special phone number, identifying real locations around Santa Cruz, where McMillen lives, and finding a buried figure of one of the game's mini-bosses. Completing it unlocked the Keeper and additional items. McMillen felt the ARG worked out but said he would not engage the community that way again, so as not to seem egotistical.

Reader's Guide

The Keeper is one of the unlockable characters in Rebirth, which with its expansions has thirty-three characters besides Isaac. Like the others, it traverses the basement and beyond, fighting monsters with tears and collecting items that change its appearance, attributes and abilities.