This character is named differently depending on the translator. Jellal is a literal translation; Gerard is a romanisation, completing the sounds that the Japanese language can't sound. His surname though, is portuguese.
Jellal is introduced under the alias of Siegrain, a young member of the Magic Council and can be considered the main antagonist of the series. His design is the same as that of Rave Master's Sieg Hart, Hiro Mashima's previous work, as a form of fan service. With his "underling" Ultear he typically disagrees with the rest of the Council, typically by showing leniency to Fairy Tail.
Though he is revealed to have some history with Erza at the start of the series, the nature of their relationship is not made clear until much later. As a child Jellal was forced to help build the Tower of Paradise. While there he was possessed by what he believed was Dark Mage Zeref, supplanting his former kind personality with a more violent one. He helped Erza to escape, took control of the Tower of Paradise's construction, and worked his way into the Magic Council as "Siegrain".
Eight years later Jellal had Erza kidnapped to be used as a sacrifice to revive Zeref. He convinces the Magic Council to try and destroy the tower in order to fuel the resurrection system. He destroys the Magic Council once their purpose is fulfilled and tries to fulfill his plans. Erza confronts him, but he defeats her and kills Simon, another childhood friend. Jellal is stopped by Natsu, who uses the very magic that fuels the system to defeat him. In the final moments before the Tower self-destructs he regains his old senses and help Erza to escape. In the aftermath Ultear reveals that she had been manipulating Jellal the entire time and that his attempt to revive Zeref was futile, as Zeref was never dead in the first place. He uses "Heavenly" magic that allows him to move and fly as quickly as a meteor and to create black holes that suck anything in. He was supposedly killed in the aftermath of his fight with Natsu, but was recently revived by the Sky Dragon Slayer Wendy. When he was revived, he had no memory except the name Erza. He found Nirvana and released the seal to destroy it. He is back to the person he was before he fell under Ultear's control, and is horrified to learn of the acts he committed. He first cast a self-destruct spell on himself, to "free" Erza of her anger. She convinces him to live on, and he helps them destroy Nirvana. He regains memory of Natsu, and provides him with flame to defeat Zero, master of Oracion Seis.
And the end of the battle, he reveals that he is afraid of regaining his memories, but Erza tells him that she'll be there for him. Before she can say more, the newly formed magic council arrives and arrests him for treason. As he is taken away, he remembers more about Erza, how he named her after the colour of her hair.
Jellal has also been shown to be able to be in two or more places simultaneously due to a very powerful thought projection spell. It was shown he was able to play as Siegrain at the council while his true self was at the Tower of Paradise at the exact same time. Mystogan is revealed to have the same face, and it is later revealed that Mystogan is the Edoras version of Jellal.
After much of the Fairy Tail guild members return 7 years later from their disasterous annual S-class mage test, it is revealed that Jellal escaped prison and, along with Ultear and Meredy, created an unofficial guild called " Crime Sorcière." To repent for their sins, Jellal seeks to destroy Dark Guilds, meeting Team Natsu while investigating a suspicious magical force. Jellal then has a private talk with Erza, resulting in her slapping him and then a kiss. Jellal becomes embarassed and lies that he has a fiancee. Ultear, Meredy, and Erza all comment on his bad lying skills.
During Grand Magic Games, Jellal disguises himself as Mystogan and competes in the competition with Fairy Tail Team B in order to investigate the suspicious magical force he felt earlier.
