Relics are unlockable advantages that can be used after your first play through. Only 1 relic can be used at a time.

Relic	How to unlock
Bigger Clips	Complete the game once with Mia in your party.
Crossbow/Assault Rifle	Complete the game once.
Faster Stronger	Complete the game once with Sheila in your party.
Item Boost	Complete the game once with Swenson in your party.
Marksman Boost	Complete the game once with Harrison in your party.
Melee Boost	Complete the game once with Jane in your party.
Silenced Weapons	Complete the game once with Blake in your party.
Stealth Movement	Complete the game once with Warren in your party.
Survivor Success	Complete the game once with Noah in your party.
Unlimited Ammo	Unlock all other relics to unlock this relic.