Oh no! You've lost an ally! Fortunately death has no permanent stay in the world of end.earth. Many merchants all over the world sell potions that awaken your party members from an otherwise permanent sleep. However, such power comes with a catch.
Potions that revive characters induce a status affliction called "revival sickness". It's sort of the ultimatum of status effects- it does damage over time like poison, as well as weakens your attack and defensive power like burn and chill. However, these effects are much more mild, and only last for three to five turns.
Some powerful items can bypass revival sickness, like the Ivory Fern pictured above, or Astral Water (as mentioned a few days ago). Because of their strength, these items are highly prized, and very rare. Other ways to revive fallen allies without sickness is by spending some time at an Inn, located in every town. For a nominal fee, all your party members can be restored to full health.