The great Hyne divided his body in two

"Hyne didn't know what to do. Out of desperation, he gave half of his body to the people and ran off with the remaining other half." - Chronicles of Vascaroon. One half was known as the "empty shell of Hyne", with strong physical power (just like ultima weapon's beam of light). This one was given to "the people" (probably to king Zebalga). The upper half was known as "Magical Hyne", with strong magical power. That half would hide exclusively in human women and turn them into witches.

Ultimecia is Rinoa in the future, dangling as a decrepit corpse from the lower part of Magical Hyne. She is being possessed because Hyne refuses to accept fate and die. This is how I understand who Ultimecia is, and her 3D model in the game