Seth is unlocked when you get soy milk and chocolate milk on the same run.
Blue Yarn - Complete the Boss Rush.
Milk Baby - Defeat Mom's Heart or It Lives on Hard.
Tiny Horn - Defeat Satan.
Seth's Hat - Defeat Isaac.
Strawberry Milk - Defeat The Lamb.
Eternal Flame - Defeat ???.
- Seth in the bible was the third son of Adam and Eve.