Illustrated Guide to the world of Spira (FFX and FFX-2)/Magical
Apparently any humanoid with very intense emotions can manage to stay behind in Spira in a shadow form that is called "unsent". Unlike ghosts, most unsent are virtually indistinguishable from live humanoids. The powers and the level of skill of each unsent seem to vary with their "age", i.e., with the time they have been unsent, and with whether or not the unsent had magical training or had been a summoner during his life. Examples of unsent characters are: Yunalesca, Maechen, Shuyin and Lenne...
Aeons are the "physical-forms" the Summoners give to the Fayth's dreams. Aeons resemble large animals or figures from Earth's mythology. There are a total of 8 Aeons in Final Fantasy X. After Yuna defeated Sin the Aeons were no longer available for Summoning but still returned in FFX2 as "Dark Aeons" who attacked Yuna and her friends.
From Wikipedia we learn that In the world of Final Fantasy X, the Fayth are the souls of the people who willingly underwent a ritual that removed their souls from their still-living bodies and sealed into stone tablets. This technique of creating Fayth is known only by those from Zanarkand. The purple hooded boy that talks often to Yuna and Tidus is Bahamut's Fayth.
Donga, Baira and Pukutak are examples of Wood spirits
Fiends are created when people die without being sent, and those without a will powerful enough to become humanoid "unsent". Fiends represent the spirit's envy and resent the living, attacking most of the time. Fiends vary widely in appearance and skills. Most fiends belong to a particular elemental class: Fire, Ice, Water or Energy
- Sand Worm FFX.jpg