Arcana, Fia

Fia Arcana (also spelled "Fear Arcana") is the daughter of a renowned Sui Rem, a sorcerer who employed magical talismans in combat and was also a master of Shadow Skills. Upon first meeting Elle, Fia resented her and viewed her as a rival. However, when news arrived that Elle had killed her father in battle, that rivalry transformed into rage. One of the Sui Rem laws dictates that when a master is slain, their disciple(s) must abandon their name until the master is avenged. Consequently, Fia changed her name to Faurink Maya ("Faury" or "Folli" for short), later earning the nickname "Plasmatizer".
Eventually, she managed to track down Elle and confront her. During the battle, Faury attempted to kill Elle using the Nothingness Talisman, a forbidden spell known only to the Arcana family, but the mine collapsed as a result of the fight, preventing this and trapping Faury under debris from which she could not escape. Surprisingly, Elle rescued her and carried her out of the mine. Faury, now beginning to doubt Elle's guilt, decided to accompany her to see what kind of person Elle was.
In the following years, Faury became good friends with Elle and often acted as an older sister figure. She viewed Elle as a child of light, loved by the gods, while seeing herself as a child of darkness who envied the light. She looked out for Elle as her conscience.
However, on one fateful day, a rumor was heard about a village turned completely to ice. The man responsible for this was Louie Francil, a fellow disciple of her father, as well as the watcher sent by the Sui Rem society to ensure that Elle and Faury finished their fight. Elle agreed to the duel, and during their preparations, admitted to Gau that she considered Faury the most powerful opponent she has ever faced. This is where the anime and the OAV diverge.
In the OAV, it was revealed that Scarface had been a witness to the battle between Faury's father and Elle, and that it was not Elle who had killed him. Confronted with this, Louie revealed his true colors as the real murderer, stating that he intended for Faury and Elle to kill each other so that he alone could claim the inheritance of his master's powers and title. Having overheard this, Gau fought against Louie and managed to defeat him, blasting him off the cliff where they stood overlooking Elle and Faury, consequently aborting their own battle.
In the anime version, Kyuo and Gau discovered Doga, the real watcher sent by the society, frozen in a river by Louie. Doga revealed that it was not Elle Ragu Louie Francil who had killed Faury's father. As soon as they learned this, Elle and Faury joined forces to defeat Louie.
From then on, Faury's friendship with Elle became even stronger. In the manga, it was emphasized that Faury was a bad cook, and it was also revealed later in the series (after the point where the anime ends) that she was also married. Her husband's name was Woaks Porelo, and it was revealed that they had married in Juliannes 4 years previous, when Faury was 15. Her reason for leaving him had been because of her father's murder, and subsequently Faury's search for Elle.