Matilda is Heart's sister, in a way. When she chose him due to a curse-turned-blessing incident, her blood and his merged perfectly. The two now merged together in blood, and mind, the two are a duo not to be messed with. Though she is dangerous, being able to shoot streems of fire up to twelve times her body length, she is the sweetest soul you'll meet within miles around. 

Matilda is a sweatheart, her kindness unique to her breed which is known for hot tempers. She is the reason Heart is so kind to others, due to her understanding his logical and analical nature, giving him proper grounds and reasons on why to do so. Heart, because of her, became who he is. While he had Arigma in his head, he has Matilda in physical form. Her personality changes the hearts of many who are around her.

While she can get hurt in her dragon form, little is said about her sword form. Her blade form is a beutiful broad sword, sharp, grand, and... indestructable by any forces. Being part of the blessing, she can turn into this for DH to use and make both of them supernaturaly powerful in combat. 

Her natural powers are of fire, which she gave part to Heart when they merged, making him able to harnes it. But she also can harnes the power he carries with him too; of light, shadow, and earth. 

Her size is just big enough to cling onto DH's back and rest there. Her weight is about 45 lbs and her wing span is about twelve feet from wing tip to wing tip. Her length, is the height of Heart if not a foot more.