Jellal was the Cheerful Child-Slave.He was courageous, steady, inspiring, and protective. I think he’s a natural leader, able to motivate a group of people to work towards a common purpose,then Ultear got to him.She changed Jellal completely.He was manipulated by what he thought to be Zeref's ghost.And then he became Obsessed with the building of the R - System.Jellal always wished for Peace.Even when he was a child as a slave,he always careful about their friends wished to live a peaceful life.Ultear found that Jellal is best option for her plan by tricking him to think that Zeref's world is where true freedom existed.