Shuyin and Lenne’s death was a cruel one but while Lenne accepted it, Shuyin didn’t. Filled with grief over their death, he refused to face his fate. His spirit was unable to reach the Farplane on its own and since all the summoners were turned into fayths by Yu Yevon, there was no one who could perform the sending.

He remained on Spira, begrudging the living. His negative feelings were impressed in one place (the Den of Woe), where pyreflies continually revived them. In a certain sense Shuyin is like a ghost, though a very Japanese conception of ghost. If you’ve ever seen Japanese horror movies (The Grudge, Ju-On, Ring for instance), you’ll notice the similarity.

According to the Japanese Shinto faith, all humans have a spirit: the reikon. After death, the spirit leaves the body and lingers between the land of the living and the land of shadows. Provided that the funeral and post-funeral rites have been performed correctly, the spirit is able to join the spirits of its ancestors. However, under certain circumstances, the reikon may become a yūrei. Yūrei are the ghosts of those who died of violent or sudden death; these ghosts are condemned to a sorrowful existence until their duty among the living is fulfilled or their soul is released through prayers.

In general, yūrei tend to stick in familiar places, such as where they died, rather then roaming around. The vast majority of this type of ghost are women whose death was caused by love-related matters. Male yūrei aren’t common and, unlike their female counterparts, they don’t hold grudges. Many of them were soldiers killed in battle; they don’t seek revenge since dying was a part of their job, yet they cannot accept their death and are therefore unable to leave the terrestrial world.

Shuyin being a ghost could also explain why he usually sees Lenne and not Yuna. Lenne and Yuna aren’t alike, yet in the Farplane he insists on calling Yuna Lenne. The Songstress Dressphere projects Lenne’s personality and since ghosts aren’t deceived by appearances, Shuyin probably sees Lenne’s spirit instead of Yuna’s body.