One of the recurring features of the Final Fantasy series are the Summons. In previous games these took the form of very powerful magic spells, things changed in the eighth installment of the series. Guardian Forces (GFs) were intrinsically linked to the Junction System which the game employed, each character required at least one GF to be attached (or set) to him or her in order to wield magic in battle.
A GF is a powerful creature which can be junctioned by a character in order to gain use of its abilities. As battles are fought, Ability Points (AP) are earned and these can be used to let the GFs learn new abilities (everything from item creation to special abilities, such as the ever-present mug).
Note, that some Guardian Forces and screenshots of Guardian Forces contain unavoidable spoilers.
This section contains spoilers! Content within this section may reveal significant parts of a game(s) story.
Shiva is one of the first two GFs found in the game and one of the most commonly occurring in the series. Named after a Hindu Goddess, the portfolio of Shiva deals mainly with Ice.
Quezacotl is the other half of the starting pair of GFs. Vaguely resembling the feathered snake it is named after, this GF replaces the more obvious choice of Ramuh for a thunder element summon.
Another classic from the series, Ifrit is Shiva's opposite dealing with Fire.