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The Lost is meant to be the most secret and elusive unlockable character in the new Binding of Isaac. The short version, with just the instructions to unlock the character, is under the spoiler: There's anyway a longer explanation tied to the character. The sequence of steps to unlock the Lost is not explained anywhere in the game and requires a long ...
Possible way to do this from CobaltStreak's stream: This was later proved several times to work and was the official way of unlocking "The Lost". Source
To get 'Man That's Fast' you need to complete the first campaign in under 12 minutes, including getting every secret and coin. To get 'Man That's Really Fast' you need to complete the second campaign in under 20 minutes, including all coins and secrets.
It turns out there is another way. With frame perfect timing you can actually go there straight from the Gordian Knot room without passing through Backsliders first. See 5:55 in this youtube video: ...
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