Jellal is a young man whose most prominent features are his blue hair and a red tattoo above and under his right eye, which he has had since his childhood.
As his Siegrain alter-ego, he dressed himself in elegant yet simple robes; consisting of a long white tunic with black stripes across the edges and on the upper part of the arms, a decorated collar, large straps connected to decorated buckles closing the inducement on the front over a dark shirt, and matching pants and shoes. He also sported a ring on his right middle finger, and usually had the medallion attesting his status as one of the Ten Wizard Saints around his neck, hidden by his jacket's collar.
Jellal himself is seen in much darker clothing, with his attire consisting of a whitish blue shirt under a hooded dark blue coat with light blue edges, two strings with shield-shaped pendants hanging down from the hood and a prominent golden symbol on the left part of the chest, reminiscent of Fairy Tail's mark, plus simple, loose, dark pants tucked inside laced boots.
Under these clothes is a special fighting suit for melee combat, designed for his Meteor spell, revealing his toned and muscular body. In the manga, this takes the form of a skin-tight outfit. Said outfit is dark around the torso and mid-arm regions of Jellal's body, whilst the rest, such as his shoulders, forearm, and thighs down, is light in color. On his shoulder and hips, Jellal has small protrusions on his outfit, which, each, come with the three holes, serving the purpose of aiding him in his aerodynamic efforts. The rest of the suit is decorated by groups of three thin lines over the arms and legs, with a double set adorning the chest. In this attire, a metal necklace with a small pendant, highly reminiscent of the Wizard Saint's one worn by his Siegrain counterpart is visible around Jellal's neck. In the anime, however, his fighting suit is portrayed as a simple skin-tight sleeveless purple shirt paired with extremely loose blue cargo pants (tucked inside simple boots), which is held up by a light bluish belt, and gloves.
Later, after his revival, he is seen taking the clothes of a defeated Naked Mummy member to cover his partially torn fighting attire (in the anime, the clothes were instead Erigor's). This new attire consists of a long black coat decorated by golden trimmings, white bands around his arms, two red medal-like ornaments on each side of his chest, a tight-fitting shirt reminiscent of the one in his old attire, and loose gray pants held up by a belt; the pants are tucked inside the boots.
Seven years later, while his physical appearance is virtually unchanged, save for his now longer, scragglier hair, his attire now consists of a dark coat, very similar to the one he stole from the Naked Mummy member, only with heart like designs running down the sleeves and sides of the coat. Beneath this, Jellal wears plated armor, which covers his abdomen, and dark pants bearing the symbol of Crime Sorcière. Over the top of all this, Jellal sports a dark, high-collared cloak.
During the Grand Magic Games, when disguised as his Edolas counterpart, Mystogan, Jellal wears the same attire that he wore during his time in Earth Land, and even opts to use the former S-Class Mage's staves as his own.
When he was young, Jellal was a very kind 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, as he created 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 (a direct result of Ultear 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.