In the anime Mythical Detective Loki Ragnarok, the character of Heimdall is a divine being based on the Norse god of the same name, reimagined within the series' unique setting. On Earth, he adopts the human identity of Kazumi Higashiyama, a young boy who presents himself as an elementary school student who has recently returned from studying abroad. He is often accompanied by a pet hawk and is of a small stature, with distinctive purple hair and red eyes, the most notable feature being that his right eye is usually hidden. This appearance contrasts sharply with his true nature as an ancient and powerful god, who is over ten thousand years old.

Heimdall's primary motivation is a deep-seated and vengeful hatred for the protagonist, Loki. He has descended from the realm of the gods to the human world with the singular goal of killing Loki. This personal vendetta stems from a past conflict in Asgard, where Heimdall lost his right eye, an injury he directly blames Loki for. The quest for revenge is not merely an order from the higher authority of Odin but has become a fierce and personal crusade that defines his entire existence in the story.

As a direct antagonist to Loki, Heimdall serves as a persistent rival. He often appears to confront Loki, but his role is more complicated than that of a simple villain. Despite his declared intent to kill Loki, his actions sometimes blur the lines between enemy and uneasy ally, a common dynamic for rivals in the series. His character is marked by a serious and often intense demeanor, driven by his all-consuming need for vengeance. However, he is also shown to be petty and calculating, willing to use underhanded tactics to achieve his goals. For instance, he once forcibly kissed the human girl Mayura Daidouji, not out of affection, but specifically to provoke psychological distress in Loki, demonstrating his cunning and willingness to exploit emotional vulnerabilities.

Key relationships significantly shape his character. His primary relationship is, of course, with Loki, defined by a cycle of hatred, conflict, and an obsessive rivalry. Beyond his hostility toward Loki, Heimdall shares a significant bond with the god Freyr, who acts as a close companion and partner. This relationship reveals a more cooperative side to Heimdall, as the two are often seen together, working in tandem during their schemes against Loki. This partnership, sometimes described as one of "heterosexual life partners," provides a balance to his otherwise vengeful personality.

Throughout the story, Heimdall does not undergo a drastic personality shift, but his rigid worldview is occasionally challenged. His unyielding hatred for Loki puts him in direct opposition to other characters, such as Mayura, whose friendship with Loki and compassionate nature stand in stark contrast to his own vengeful path. His development is often shown through these interactions, where his single-minded focus on retribution isolates him from the more nuanced moral landscape of the human world.

In terms of abilities, Heimdall possesses powers befitting his divine heritage. His most iconic trait is the power of his right eye, the very eye that Loki stole. Even in its absence or damaged state, this eye is a source of significant magical power, contributing to his strength in combat and allowing him to use various spells. He has also demonstrated the ability to generate and manipulate fire, adding an offensive edge to his capabilities. His human disguise as a young schoolboy is a form of magical concealment, allowing him to move undetected in human society while he plots and waits for his chance to exact revenge on his eternal enemy.