In Final Fantasy IV, Titan comes to Rydia’s aid after she cries out “stay away” and “leave me alone! I hate you!” Leviathan appears sometime after Rydia feels fear on the Ship to Mysidia. Why do they come from? When we control Rydia, and she knows black and white magic. Yet she can’t summon Eidolons at will. This is because she can’t control her emotions. When she feels fear, rage or stress, Eidolons suddenly appear. They can come to her aid without her having to directly summon them. Perhaps her latent powers activate subconsciously, through her emotions. But in all of the Final Fantasy games, emotions affect summons.
Terra can activate the latent powers of Esper side after Valigarmanda transforms her. She’s confused, fearful and amnesiac, but after the battle at the Narshe Snowfields, she becomes wild. So what’s going on here? In Terra’s case, her markedly disturbed sense of identity causes her intense and uncontrollable emotional reactions. It isn’t until after Ramuh explains Terra’s state that the party is able to control Eidolons. And once Terra regains her senses, she’s able to control them as well. The second time she loses control of her Esper side is fighting Humbaba in Mobliz.
Also consider Kefka, who gains enough magic power from the Warring Triad that he becomes an Esper God himself. However, while he was already a bit crazy, he completely loses his mind during the World of Ruin. His erratic nature causes him to further destroy the world in his god-like state. As with Terra, Kefka’s loss of emotional control causes him to lose control over his powers.
But the most infamous example comes from Final Fantasy VIII:
A GF is an independent energy force. By combining it with para-magic, it is possible to control tremendous energy. Memory loss is a possible side effect, but this has not been proven as of yet.
But we actually know that this is true. The intense or uncontrollable emotional reactions to Guardian Forces can cause memory loss. Of course, the memory SeeD loses isn’t due to the rational part of their brain – it’s the emotional one. Squall and others have trouble remembering events from their childhood. The bonds they shared with one another sever due to their use with the Guardian Forces.
These mystical entities reveal themselves before only to a select few l’Cie. It is said that they are saviors, come to rescue hopeless l’Cie who find themselves bound to a Focus against their will. If this is true, they offer a brand of salvation few indeed would seek willingly: without exception, Eidolons attack the l’Cie whose presences they grace.
In Final Fantasy XIII, Eidolons appear before the l’Cie when the party receives high levels of stress. In this sense, the Eidolons focus are to rescue the hopeless. Yet, they can only do so once the l’Cie’s stress levels heighten.
In all four examples, emotional turmoil, stress and angst can cause power levels to fluctuate.