Animus Codex is used to uncap or characters instead of using treasures. One codex is consumed to uncap one time.
The most efficient use for this item is for uncappingcharacters to 5★. Uncapping characters to 5★ typically costs . Large amounts of Blue Sky Crystals are also used in various weapon series upgrades.
Another option is to uncap Story Characters to 4★ (except Lecia and Cain). Fully uncapping them requires , which are required to upgrade Sierokarte's shop in order to fully-forge Revenant Weapons. Fully upgrading the story characters to their level cap rewards . If Mithra Showdown is too difficult for you, then using a codex to uncap is an option, but not as efficient.