Eres can only be used by specific people called Erens (爪術士 Soujutsushi?). The terminology comes from a Relares word meaning fingernail, as the use of eres power causes the user's nails to glow. There are two types of eres: Iron Eres (アーツ系 Aatsu Kei?, "Arts Type") and Crystal Eres (ブレス系 Buresu Kei?, "Breath Type"), which replace Strike artes and Magic artes, respectively. All playable erens can only use either Iron Eres or Crystal Eres. No protagonist has access to both, although several enemies do. As the avatar of the Nerifes, the Merines has the power to rescind the power of an eren, or even the power of every eren in the world.
Iron Eres are learned through normal experience levels and mastered by repeated use. Mastering different Base Eres (特技 Tokugi?) can sometimes unlock advanced eres combinations, which are called Arcane Eres (奥義 Ougi?). An Iron Eren can also create new techniques unique to their fighting style by purposefully combining their techniques into new skills known appropriately as Compound Eres (我流奥義 Garyuu Ougi?, "Own Style Arcane Eres"). Compound Eres are unique to each character.
For example, Senel Coolidge's Compound Eres act as throws for special enemy classes, while Chloe Valens's act as counters to attacks. Crystal Eres scrolls are unlocked by Crystal Erens through normal experience levels, but unlike Iron Erens, who must use their eres in order to master them, Crystal Erens must master their eres before using it. They accomplish this by gathering "Eres Stones", the crystallized life force of monsters, which enables them to access the power within the eres scrolls. Eres Stones are gathered by defeating enemies of a certain race.