Frey, also known as the Mysterious Thief Freyr, is a character from the anime Mythical Detective Loki Ragnarok. He is the older brother of the goddess Freya and, like his sister, is a member of the Norse pantheon who has been exiled to the human world, or Midgard.

In the human world, Frey appears as a tall, dashing young man with brown hair that reaches his neck and blue eyes, often seen wearing a cape that complements his flamboyant persona. He is initially brought to Earth with the mission to assist the god Heimdall in destroying the trickster god Loki. However, Frey quickly abandons this primary objective upon discovering that his sister, Freya, is also present in the human world.

Frey is characterized by an impulsive, quixotic, and highly dramatic personality. He perceives nearly every situation through the lens of romantic clichés and absurd non-sequiturs, acting as a stark contrast to the more grim and focused Heimdall, who becomes his roommate. Rather than displaying a genuine, serious drive to eliminate Loki, Frey seems more content with rushing into situations to make a grand, heroic rescue or causing a spectacular blunder. He is notably ignorant of many key details, such as the fact that Loki has been reincarnated in the form of a young boy.

His motivations shift significantly over the course of the story. While his original purpose was to destroy Loki, his primary agenda quickly becomes locating Freya and ensuring she is not being corrupted by Loki's influence. To capture Loki's attention as a natural enemy to a detective, Frey adopts the persona of a master jewel thief, becoming the "phantom thief" that any great detective would be compelled to pursue. His plans take another dramatic turn when he encounters the human girl Mayura Daidouji. He falls in love with her at first sight, dubbing her his "Yamato Nadeshiko" or "Classic Japanese Beauty," and he often chases after her, declaring himself Loki's rival for her affection, though his romantic efforts are consistently hopeless.

Key relationships define much of Frey's role in the narrative. His relationship with his sister Freya is typically depicted as chaotic and comedic, often involving her hitting him with a metal basin for his antics, which has led to them being called the "moronic siblings". Despite his ineptitude, he does show a caring, if overbearing, side for her. In contrast, he teams up with Heimdall in his anti-Loki efforts, but Frey seems to treat their goal more as an amusing pastime than a serious crusade, while Heimdall does not appear to be aware of Frey's changing priorities regarding his sister and new love interest.

In terms of abilities, Frey is presented as a god, though his specific powers are not extensively detailed beyond his capability to carry out flamboyant heists and perform heroic rescues. As a phantom thief, he is skilled in dramatic entrances and escapes. He and Heimdall also possess the ability to use runes; Heimdall uses one to cast a debilitating spell on Loki, implying Frey likely has access to similar divine magic, even if he is more focused on his romantic and familial dramas. Ultimately, Frey is a character driven by impulse and romantic idealism, whose primary developments involve abandoning his initial mission for personal ones centered on love and family, all while maintaining a cheerful and dramatically oblivious demeanor.