Origin

The Valkyrie was a persona adopted by the Enchantress during a ploy to split the Avengers along gender lines- leading a force of the Wasp, Black Widow, Scarlet Witch against Black Panther, Iron Man, Captain America and Hawkeye / Goliath. Presumably it was the as-yet unretconned god level Bendis-powers of the Scarlet Witch and the Enchantress' own magics that allowed the fight to be less one-sided than one might otherwise have thought.

The actual Valkyrie was an alter ego character somewhat similar to the Hulk or Captain Marvel except that the physical form of (originally) Barbara Norriss was used as a focus by the Enchantress for a dastardly scheme to entrap Brynhilde / Brunhilda / etc. that resulted in a mad Barbara Norriss gradually reasserting her neurotic persona over that of the dynamic Defender.

Fresh from untangling that ungodly (pun intended) origin the Valkyrie's past was once again entangled in too many layers- too many cooks spoiled the broth.

Valkyrie was essentially an Asgardian, either the spirit of an Asgardian charging a mortal god with power somewhat like the original proper version of Thor, and at other times, especially later, as a fully fledged godling of Asgard, able to exchange her appearance and costume by drawing her sword Dragonfang.

Varying Marvel writers have emphasized or de-emphasized the Asgardian-ness of the character, as is often the case at Marvel especially in recent years there have been changes for changes' sake rather than any attempt at consistency.

This is her origin as told by the sentient, disembodied eye that once belonged to Odin:

Brunnhilde was selected by Odin, the ruler of Asgard, to lead the Valkyrior, the "Choosers of the Slain". It was their duty to go to the battlefields of the mortals who worshiped the Asgardian Gods and to choose and bring whoever was worthy to Valhalla, the land of the honored dead. She was found to be quite capable of remaining in this position of power for centuries.

But when Odin was forced to give his son, Thor, the mortal identity of Siegmund he was dismayed when circumstances forced him to declare that Siegmund must be slain.

Brunnhilde sensed her lord was decreeing this against his true wishes and so she tried to protect Siegmund but Odin took it upon himself to kill him and she could do nothing to stop him.

So instead she helped the pregnant lover of Siegmund, Sieglinde reach safety. Because of this defiance Odin stripped her of her immortality and godly powers and cast her into a trance.

She was later awakened by Siegfried (a mortal incarnation of Thor), the son of Siegmund and Sieglinde and the two became lovers. He eventually betrayed her and was murdered but in love with him still she jumped onto his burning funeral pyre, dying with him.

Odin then decided to restore both of them to life, restoring their godliness and wiping their memories of their mortal lives.

She is a young woman and she owns a black Pegasus (a winged horse)and a huge divine sword.

Friends and Lovers

The Vakyrie has been romantically linked with the ex-husbands of at least two of her Earthly hosts, although never voluntarily. During an inconsequential Marvel Team-Up she seemed to be rekindling some ancient tryst with the Mighty Thor (they were lovers in a past life-- yes, really) but that was not seriously pursued despite the many opportunities it would have presented for classic soap opera. Perhaps that would have been too "DC" for the new version of what was once the House of Ideas.

The Valkyrie has also formed many close affectionate bonds in a sisterly or even motherly way with her Defenders team mates, notably Hellcat (sisterly) and Cloud (maternal).

Valkyrie, when written well, is a wonderful addition to the Marvel Universe, and a welcome sophisticated edge for otherwise bland stories. Written badly, she serves to show in glaring contrast all that has gone so very wrong with modern Marvel.

Ultimate Valkyrie

The Ultimate Valkyrie was originally just another non-powered member of the poseur-ful Ultimate Defenders-- a hot, but not too bright, nineteen-year-old with a superhero fetish. She had a brief affair with Hank Pym when he was a member of the group. Her abilities were limited to physical fitness and minimal martial arts training.

Later, she shows up in the company of Thor with a magic sword, a winged horse, and apparently real powers and combat skills. Her bimbo personality is still intact, however.