Mathilde Saint-Omer is the previous owner of the title "Flame-Haired Burning-Eyed Hunter". She is shown in the flashbacks leading to the ceremony of Shana's ascent as a Flame Haze. She was Wilhelmina Carmel's friend and companion. The two were regarded as the greatest pair of Flame Haze, rivaling in power and referred to as a counterpart to the "Pair of Wings", Crimson Lords Merihim and Illuyanka. She was in love with Alastor.

(Source: Shakugan no Shana Wiki)

He had thought his normal days would last forever, until they were ended abruptly by an encounter with a girl with red hair and eyes. He named her Shana. 
Not long after he enters high school, Sakai Yuji&rsquo;s normal days come to an end. He sees something bizarre on his way home: people are frozen in a strange fire, and a monster that looks like a large doll is attacking them. Just when he's about to be killed, a girl in a dark cloak wielding a large sword appears, and slices the monster in half. Although Yuji survives the strange encounter, the girl informs him of a desolate truth: " 'You' don't exist anymore." 
She calls herself a Flame Haze who hunts "Guze no Tomogara"; intruders from another world. She explains to him that he is a "Torch", a substitute of Yuji whose &ldquo;existence&rdquo; has already been devoured. She also told him that he is a special Torch, "Mystes", which contains a precious item in it. When he looks at his chest, he sees within himself a faint flame, just as she said. 
It is in this way that his eternal struggle alongside the girl begins. 
(Source: AnimeNfo)

Continuing from the events of the last series (Shakugan no Shana II), both Shana and Kazumi stand at their respective locations waiting for Yuji to meet with one of them, only to discover that Yuji has disappeared, with not even the slightest evidence of his existence left behind. All is not lost however, as the letters the two of them sent to him still give hope of Yuji's continued existence. As Shana and company search for answers on Yuji's whereabouts, they soon find the truth staring back at them when Yuji reappears in front of them as the leader of Bal Masqu&eacute;. Left with little choice, Shana must now confront her most unlikely adversary on the battlefield in her toughest challenge yet.
 
(Source: Anime News Network)

Shakugan no Shana X Eternal song -Harukanaru Uta- is based on the events of the tenth Shana novel. 
The events in Eternal Song focuses on the events of the Great War, with a certain focus on the lives of two Flame Haze, the previous "Flame-Haired Burning-Eyed Hunter" Mathilde Saint-Omer and "Manipulator of Objects" Wilhelmina Carmel. They were famous as the counterpart of the Crimson Lord duo called as "Pair of Wings" ("Rainbow Wings" Merihim and "Iron-clad Dragon" Illuyanka). Their adventure starts in the middle of the Great War, an event of great turmoil between Crimson Denizens and Flame Haze and a period of great activity between the two factions.