Diemia; Guardian of Stone, serves as an attack summon for Iris. Inflicting physical damage on enemies in a straight line. It is acquired after defeating the optional boss Rockwell. It can be fought as early as Chapter 5 of the main story. It's look has completely changed from the previous 2 games. Instead he now looks like a Rhino made of stone. It also serves as a blade outfit for Nell. Giving her powerful lance attacks.
There is also a glitch in using Diemia where if you use it during a burst mode it'll inflict much more damage than it normally should. Due to this glitch Diemia is arguably the strongest Summon for Iris to use in the game. Making Fanatos the 2nd strongest for Iris to use. It is possible for Iris to inflict over 10,000 damage in one attack through this glitch.