There is an unlockable character called Eden which appears to require Eden tokens to unlock. How do I get tokens to unlock him or her, and how do they work?
Eden is unlocked when the player defeats The Womb for the first time. Eden is a new character in The Binding of Isaac: Rebirth. Every time you beat Mom's Heart, you will get another Eden Token.
Basically you unlock Eden for defeating Mom's heart. Mom's heart can be found in The Womb, and Mom's heart can be defeated only once per run. Each kill of Mom's heart gives 1 Eden Token, which gives one run with Eden.