Hresvelgr is a Frame Arms Girl, a small autonomous robot, who appears as a central character in the anime. Her name and design are inspired by the mythical giant eagle from Norse mythology, known as the corpse-swallower who creates the world's wind. Sent to the home of the main human character, her primary directive is to engage in combat with another Frame Arms Girl named Gourai to collect valuable battle data for the company that created them.

In personality, Hresvelgr is defined by her straightforward and intense love for battle. She is depicted as an innocent and honest character who makes no effort to hide her feelings, particularly her excitement for a good fight. Her greatest pleasure and primary motivation is engaging in combat with Gourai, whom she considers her rival. This singular focus can make her seem a bit like a street punk or a delinquent in her approach, as she is not above challenging others directly and without preamble. When she first appears, she is confident, boasting of being the strongest, and her actions are driven almost entirely by the pursuit of a satisfying battle.

Hresvelgr’s role in the story is initially that of an antagonist and a formidable rival. Her arrival dramatically changes the status quo when she defeats Gourai, handing the protagonist her first-ever loss and forcing her to seek improvements. This event is a crucial turning point, as it pushes the other characters to grow and evolve. Her key relationship is undeniably with Gourai. What begins as a straightforward mission to defeat her rival deepens over time. After a series of confrontations, their bond transcends that of mere opponents, and they develop a genuine friendship.

Throughout the narrative, Hresvelgr undergoes a significant development. After her initial defeat of Gourai, she herself returns to the factory for a powerful upgrade, transforming into a new form called Hresvelgr Ater, also known as the "Shiroi Hresvelgr" or White Hresvelgr. This upgraded state enhances her abilities but also alters her artificial personality to prioritize combat above all else, stripping away much of her emotional expression. This modification proves to be unstable, as she loses control and enters a berserk state during her next battle with Gourai. It is only after being defeated by Gourai, who had been temporarily powered up with parts from other Frame Arms Girls, that she returns to her original, more balanced self. This experience solidifies her connection to the group, and she comes to live with them, showing a more well-rounded side by participating in social activities like a Christmas dance performance.

In terms of notable abilities, Hresvelgr is considered one of the most heavily armed and powerful of the Frame Arms Girls. Her standard equipment is vast, including a sword-rifle, a beam launcher, and a variable戟 (a type of halberd). She is highly proficient in aerial combat, capable of high-speed flight and easily outpacing land-bound opponents. Among her specific weapons are the TCS Oscillator, a powerful beam weapon, the Verrill Shot Launcher, and the Beryl Smasher. Her special techniques include the devastating Tornado Typhoon Cyclone Blade, which she used to secure her first victory over Gourai. Her upgraded form, Hresvelgr Ater, features a new visual design and even greater combat specs, though at the cost of emotional stability.