Hild is the leader of Hell; her official title is Daimakaichō (大魔界長, literally translated as Great Demon World Leader). She is, to date, the most powerful demon in Niflheim, although she is currently rivalled by her daughter, the half-demon half-goddess Urd.
She is first introduced when Peorth returns to Tarikihongan, telling the others that from her previous scuffle with Welsper, she hasn't been able to find a way to revert to her normal age. They finally realize that the only way to undo the curse that Welsper put on Peorth, they needed to summon the leader of Hell to return the goddess to her original form.
Later, with a rebellion caused by Hagall, she had been overthrown from the position of Daimakaichō and had been imprisoned. However having left her child form on Earth, Hild devised a counter-plan, which she enacted by sending the Norns and Keiichi to Niflheim. Due to Keiichi's initiative, a backup, which she had split of off herself and left with him, was capable of tacking control of Hagall and releasing Hild's real body from her prison.
Hild, at first impression, is a very cheerful and happy-go-lucky person. However, under that exterior, she is very manipulative, calculating and merciless. After she leaves to Niflheim after restoring Peorth's adult-form, she eventually returns in the form of a miniature of herself. Much like how Belldandy and Urd can split into many miniatures of themselves, Hild has the ability to leave a fraction of herself behind in the form of a little girl. She continues to wreak havoc in this way.