Equipping a Geo allows you to summon an Ultimate Form Spectrobe. This replaces your combo attack, and after you use it, the battle ends. Their use automatically terminates vortexes, and they cannot be used in Sequence Battles. Since they all have the same effect, the main factor in deciding which one to use is personal preference.

Unlockable	How to Unlock
Cyclone Geo: Zorna	Beat Sequence Battle lvl. 1.
Flame Geo: Tindera	Automatically unlocked during the battle following the one against Dolgon and Dolgan.
Hammer Geo: Larrup	Beat Sequence Battle lvl. 3.
Ice Geo: Artezza	Beat Sequence Battle lvl. 5.
Plasma Geo: Voltorn	Beat Sequence Battle lvl. 4.
Thunder Geo: Fulvina	Beat Sequence Battle lvl. 2.
Wing Geo: Shulla	Beat the last Sequence Battle (lvl. 6).