Enlightened rule by the dead is preferable to the misguided failures of the living.
Unsent (死人/死者, Shibito/Shisha?, lit. Dead person/Departed) is a state of being in Spira in Final Fantasy X and Final Fantasy X-2, and the common name for those in that state. When a person dies his or her spirit enters the Farplane, either willingly by having accepted death, or with the help of a summoner performing a sending. If a person's spirit does not enter the Farplane it will remain on Spira as an "unsent", where the person's pyreflies reconstruct their body, usually in their old image. The unsent can still send their own souls no matter how long they have lingered and their bodies will disperse into pyreflies.
Unsent are typically overcome by malice and often begin to resent the living, eventually becoming fiends, though some unsent retain a corporeal state and a rational mind; this is associated with the unsent who have a strong conviction of some kind, such as a "mission" they feel compelled to complete. Some unsent can adopt the shape of a fiend at will, and absorb others' pyreflies to grow stronger.
Different unsents' bodies take different forms. Some unsent spirits remain identical to how they were in life, but some go through changes, and may appear to age even when biologically immortal. Shuyin is a different type of unsent, as his pyreflies do not form a physical body, instead, the pyreflies are only "imprinted" with his hatred and act on their own and can possess the bodies of the living. Final Fantasy X-2.5 ~Eien no Daishō~, a companion novel and sequel to Final Fantasy X-2, describes how the deceased may remain conscious through their own death and have a modicum of autonomy on deciding to either "descend to hell" (might simply refer to the Farplane) or remain and become a fiend.
Once the deceased has entered a Farplane they usually must remain there, but there are known to be exceptions. Jyscal Guado's spirit emerges from the Farplane to deliver Yuna a sphere recording, assuming a form akin to an unsent even if he was sent upon his death. In Final Fantasy X-2.5 ~Eien no Daishō~ and Final Fantasy X -Will- other spirits return from the Farplane as well in a phenomenon known as the beckoning.
The concept of the unsent in Final Fantasy X is similar to the Japanese folklore of the yūrei, which may have inspired their design. According to traditional Japanese beliefs all humans have a soul, and upon a person's death the soul leaves the body and enters a form of purgatory, where it waits for the funeral and post-funeral rites to be performed so that it may join its ancestors.
However, if one dies in a sudden or violent manner, such as murder or suicide, if the proper rites have not been performed, or if they are influenced by powerful emotions—such as a desire for revenge, love, jealousy, hatred or sorrow—the soul transforms into a yūrei, which can bridge the gap back to the physical world.
The yūrei exists on Earth until it can be laid to rest, either by performing the missing rituals, or resolving the emotional conflict that still ties it to the physical plane. If the rituals are not completed, or the conflict left unresolved, the yūrei will persist in its haunting. Yūrei are frequently depicted as being accompanied by a pair of floating flames or will o' the wisps that are separate parts of the ghost rather than independent spirits. This is similar to how the unsent in Final Fantasy X are related to pyreflies.