Valkyrie is one of the signature heroes of City of Heroes, although there is relatively little info on her compared to her teammates.
Valerie Kellum was an archeologist who found a mysterious spear in Scandinavia. After some inspection she discovered that the spear was in fact highly technological in origin. This attracted the attention of Crey, and during a struggle with their thugs the spear was activated and Valerie was injected with nanites. These gave her both advanced fighting abilities and a different personality.
So far she has not had any comic appearances, even in the recent Vindicator issues.
Valkyrie is the trainer for Steel Canyon, near her mentor Positron. While she does not yet make any fighting appearances in the game, her Praetorian counterpart Battle Maiden fights using a sword due to the lack of a spear power set.