Mathilde is Alastor's previous flame haze.
Seems like her name came from a France. Saint Omer is a name of a place in France. Or, Mathilde de St. Omer, Lamert II, Comte de Gand, Seigneur de Bornhem's wife.
Seems like she has black eyes and hair, but they transformed into red in her battle form. She wore a gown with metal pieces on her chest. She wore a hand accessory, which is her contractor vessel called Cocytus. The shape if the hand accessory is the same like Shana's pendant.
She has quite a strange personality, became a flame haze on her own desire. This made her having no friend, because others thinks it's weird. The only want that accepted her is Wilhelmina Carmel and Tiamat, even though they didn't understand her.
Mathilde's primary abilities are the same as Shana's, the ability to summon flames and use it to empower their weapons. Aside from this, Mathilde can summon a flaming horse which she rides whenever she and Wilhelmina travel.
She also uses the Unrestricted Spell Knights (騎士団 Kishidan, Order of Knights) to summon an army of armored warriors which she uses to defeat a large number of opponents. This Spell can also be used with another, called Hajoutsuchi (破城槌 Hajoutsuchi , Castle-breaking Hammer) which summons a large battering ram.
One of her most powerful attacks, Vorpal Phalanx (大殲滅 密集突撃 Daisenmetsu Misshuutotsugeki, Grand Massacre - Close Assault), strengthens her attack by utilizing "Knights" to pierce the area where the attack occurs, allowing Mathilde to cleave through stronger opponents.
In the Second Great War, she fought an organization called Toten Glocke. It's leader, Asiz, wanted to use "City Devourer" to fulfill his desire, that can distort the world's balance. She then tried to stop him, and the only way is to use Tenpa Jousai, a greatest and most dangerous Unrestricted Spell possessed by "Flame-Haired Burning-Eyed Hunter". It summons Alastor to this world, and it consumes great power of existence from the opponent (Mathilde sacrificed a crimson lord). The usage can be fatal, because Alastor's existence is extremely big, and if the container's capacity is not large enough, the container will die, which happened to Mathilde. Before her death, she confessed her love to Alastor and sang a love song for him.