Quote Originally Posted by MagicSpice View Post
even still, in terms of defense, tiamat should also be better than reiki... unless it's fire damage you take

reiki gives fire resist (more than jack but for 3 turns only as opposed to the entire fight), tiamat gives a flat HP barrier... this means tiamat is less situational than reiki int he case of being in sub slot of for using a water team in an off-element situation
Reiki's buff is only 1 turn.

If Jack or Reiki aren't being used as a main eidolon, I would not even use either of them as a sub. Unlike Tiamat, the other two have fairly average/low atk, so there are better eidolons out there to use even if different element.