When he was young, Jellal was a kind, compassionate boy, and cared deeply for his friends. He was always cheerful and optimistic, despite his disposition as a slave. After a failed escape attempt, he readily accepted punishment in Erza's stead, and never once lost his faith in the thought of being free.

 

Later, as he was manipulated by what he thought to be Zeref's ghost, he drastically changed; Jellal became obsessed with the idea of building a new R-System and treated the slaves much better — to the extent that they didn't even think they were still slaves. But through this method of treatment, they did more work, which was Jellal's ultimate reason for treating them as such in the first place. He sees his actions as a game, even if said game results in the loss of lives, his very own included.

 

After he was revived by Wendy Marvell, he suffered from amnesia and seemed to be somewhat afraid of the world. Though not remembering his own identity, he remembered the names Erza and Natsu Dragneel's names, respectively. As he learned of his sins, Jellal wished to die, as he believed it to be his only form of atonement, but Erza convinced that through living, he can find a way to make up for his past horrid deeds. Following these instances, even when arrested by the Knights and knowing he would be given the death penalty, Jellal remained calm and even happy, just as he did when he was a child. Mavis Vermilion even commented that Jellal possesses the same "heart" as those in Fairy Tail, in spite of the fact that he isn't an actual member.

 

Seven years later, after regaining his memories and having returned to his original personality, Jellal is now a calm, mature, intelligent, and kind individual. He has shown a cheerful side, as he often smiles when pleased, or when his friends are happy. While as "Mystogan", he makes sure he acts quiet and aloof, so as to not arouse suspicion to the fact that Fairy Tail is housing an escaped convict. However, Jellal's regret towards his past deeds have caused him to feel the need to atone for his sins, which he did by creating Crime Sorcière: an Independent Guild dedicated to the destruction of Dark Guilds. Despite this, he still feels he deserves punishment, as he told Ultear and Meredy that "punishment" is the theme of their guild. It is this guilt that keeps Jellal from expressing his love for Erza, which she reciprocates, as he believes he doesn't deserve her. 

 

Despite his rather serious nature, Jellal does have a comedic side, as in his fight with Jura Neekis, where (as a direct result of Ultear and Meredy sabotaging his battle with the Wizard Saint) he has fits of hysterical laughter, as well as reacting strongly to the sensation of peppers he had in his mouth. Later, when he walked back to his teammates, Jellal felt ashamed, and stated that he had no words to describe his feelings of shame and embarrassment.

 

As a child, Jellal was forced to help build the Tower of Heaven together with other slaves, such as Erza, Shô, Simon,Wally and Millianna, wherein he acted as the leading figure of their group.

 

There, after a failed escape attempt and the reclusion that followed it, he waspossessed by what he believed was the legendary Dark Mage Zeref, supplanting his former kind personality with a more violent one, which made him almost completely insane.

 

He exiled Erza from the island and took control of the construction of the Tower of Heaven in order to resurrect Zeref, and thus create a world of "true freedom". He bombed the ship that was supposed to take the slaves off the island, and told them afterward that it was Erza's doing, as she had gone insane with her newfound Magic Powers. Believing that she had truly escaped on her own and thus betrayed them, the slaves helped him build the tower for the next eight years as a way of showing their gratitude for saving them from certain death.

 

While growing up, Jellal was taken under the tutelage of the Oración Seis leader Brain, having taught Jellal some of his skills and Magic, namely one of them being the Self-Destruction Spell.

 

Meanwhile, Jellal had worked his way into the Magic Council using a Thought Projection of himself, named "Siegrain".Sometime after this, Jellal, as "Siegrain", ran into Erza Scarlet, and prevented her from attacking him by lying to her, saying that he was Jellal's twin brother.