Ellone is a mysterious girl and the missing "Sis" of Squall's past. She has the ability to send a person's consciousness back in time and into the body of another, so they can experience the actions of that person. She uses this talent to send Squall's party into Laguna's past adventures, hoping that they would alter the past; however, she eventually realizes that her abilities can only view history, not alter it. Ultimecia needs this power to achieve "Time Compression", so she uses Edea and the Galbadian military to find her.
Source: Wikipedia, "[en.wikipedia.org/wiki/Characters_of_Final_Fantasy_VIII%23Ellone Characters of Final Fantasy VIII]", available under the CC-BY-SA License.