Mystogan, born under the name Jellal, was the son of Faust and the prince of Edolas. He was seriously injured at one point during his childhood, but was saved by Panther Lily, an Exceed. Though Panther Lily was exiled from Extalia for assisting a human, he and Jellal became close friends. Over time, Jellal grew discontent with his father's use of the inter-dimensional spell Anima to steal Magic Power from Earth Land, a world parallel to Edolas which had a limitless supply of Magic with which Faust used to replenish the limited resources Edolas had. This, in turn, led Faust to use that Magic Power to conquer other regions in Edolas and expand his kingdom, growing increasingly mad with power. Hoping to end his father's senseless campaign, Jellal left Edolas through an Anima portal and entered Earth Land in the year X777 (Earth Land time), traveling the land under the name Mystogan in search of more portals to close them.

During his travels, Mystogan encountered a young girl named Wendy, who had been abandoned by her guardian, the Sky Dragon Grandeeney. He traveled aimlessly together with Wendy for about a month (using his real name "Jellal") when he suddenly sensed the presence of Anima. Thinking it would be unsafe to let Wendy travel with him any further, he placed her under the care of Roubaul, an old man who was living nearby, telling her he would leave her in a guild; as Wendy would later discover, Roubaul was not truly a guild master, but whether or not Mystogan realized that is unknown.

Soon after, Mystogan suppressed Anima and eventually joined the Fairy Tail Guild, becoming an S-Class Mage in X782. Aware that there was a counterpart of himself in Earth Land, he remained distant from the other guild members and covered much of his face behind a mask to keep his identity a secret, particularly from Erza Scarlet, who had a troubled past with the Jellal from Earth Land.

Source: fairytail.fandom.com/wiki/Mystogan