Heimd is the human disguise and earthly incarnation of the Norse god Heimdallr from the anime Mythical Detective Loki Ragnarok. In the mortal realm, he adopts the name Higashiyama Kazumi and presents himself as a young boy who has recently returned from living abroad. Despite his childlike appearance, standing only 102 centimeters tall, his true age surpasses ten thousand years, creating a striking contrast between his innocent facade and his ancient, divine nature. He is frequently accompanied by a hawk, and his schedule includes regular attendance at a cram school, which serves as part of his cover story for existing in the human world.

As the god of light and dawn, Heimd embodies qualities of watchfulness, purity, and a sense of justice that aligns with his original mythological role as the guardian of Asgard. His personality is marked by a serious and determined demeanor, driven by a deep-seated grudge against the god Loki. His primary motivation for descending to the human world is to exact revenge for the loss of his right eye, which he believes Loki stole from him. In the context of the anime, this personal vendetta against the series' protagonist forms the core of his character arc and his reason for becoming involved in the earthly narrative.

Within the story, Heimd functions as a formidable antagonist whose actions are directly tied to the overarching conflict between the gods. His role is that of an avenger, a divine being who has taken on a mortal form specifically to confront and eliminate Loki. This pursuit leads to a significant episode in the series, titled The 21st Night: Heimdall, God of the Dawn, where his awakening as a god signals a major escalation of events and a direct threat to Loki and his companions. The appearance of Heimd forces a confrontation that tests Loki's understanding of his own past actions and destiny.

His key relationships are defined by this central conflict. He is the self-proclaimed enemy of Loki, and their opposition stems from a mythological precedent, as the original Heimdallr and Loki were fated to kill each other during Ragnarok. In the anime, this age-old animosity is personalized through the specific act of him losing an eye, a direct echo of the mythological accounts where Heimdall possesses exceptionally keen sight, a power that here was taken by his rival. This singular focus on revenge largely isolates him from other characters, as his entire purpose revolves around his mission to enact retribution.

Regarding his development, Heimd begins as an antagonist consumed by a single-minded need for vengeance, rooted in a past transgression. However, the narrative reveals a crucial layer to his motivation: the truth that his eye was not stolen by Loki in a simple act of malice, but was actually taken by Odin, the All-Father. This revelation complicates his simplistic view of Loki as the sole perpetrator and suggests a deeper manipulation of events among the gods, shifting his personal quest for justice into a more nuanced tragedy where he has been aiming his anger at the wrong target.

In terms of notable abilities, as an incarnation of a god, he possesses powers and a true form that far exceed his small, human disguise. He is intrinsically linked to concepts of light and dawn, and in his divine state, he is considered the watchman of the gods, gifted with incredibly acute senses that allow him to perceive events from great distances. While in the anime he primarily pursues his vendetta, his mythological counterpart is famously armed with the horn Gjallarhorn, which he would sound to announce the beginning of Ragnarok, symbolizing his role as a herald of critical moments.