Ellone is one of the minor characters of FF8, but she plays a big role in the game. Ellone was adopted by Raine after her real parents had died and stayed there until her kidnapping by Estharian soldiers. She also calls Laguna "Uncle Laguna" since he was around in Winhill for so long. She loves Laguna dearly, like a second father. Ellone was sent to Edea's Orphanage along with baby Squall when Raine had died. She acted like an older sister to Squall in a sense while everyone called her "Sis" because she was the oldest out of the orphanage gang.
She seems to be your ordinary girl, but Ellone possesses an ability that many evils in the game want to gain: the ability to go back in time. She can send people that she knows into the subconscious of another person that she has met in the past. This was how Squall and the gang were able to see Laguna's experiences through his eyes, and this was how everyone was able to realize Ellone's power.
Esthar, under Sorceress Adel's rule, had a great interest in Ellone, particularly the eccentric scientist Odine. Somehow, he was able to study Ellone enough to create a time machine that would let someone possess the body of another in either time era. This was how Sorceress Ultimecia began an interest in Ellone and tried to search for her.
I guess you can say that she's a quiet person, but not much is known about her except for the information above.