WARNING: The following contains spoilers for Episode 6 of The Detective Is Already Dead, "Crimson Devil, Ice Queen," now streaming on Funimation.
Every iconic pop culture detective has their own archenemy who mirrors them in some capacity but also challenges the detective in profound and personal ways. In some cases, they even present a twisted version of the detective’s own goals. Some iconic archenemies include Sherlock Holmes' Professor Moriarty and Batman's Joker. But who does Siesta have as an archenemy?
Episode 5 of The Detective Is Already Dead provided more backstory on Siesta and her years working with Kimihiko Kimizuka prior to her death. Since Episode 1, Siesta had been tracking the criminal activity of a mysterious organization known only as SPES, which seemingly specializes in creating cyborgs with enhanced senses. Two of its known members include Bat (who has enhanced hearing) and Cerberus (who has a keen sense of smell). The same episode also introduced Hel as the supreme leader of SPES, who has proved themselves quite formidable against Siesta. Episode 6 further fleshes out Hel's villainous motivations.
Little is known about who Hel is as a person, where they come from or if they even joined SPES willingly. In terms of personality, Hel is highly intelligent and a master strategist truly capable of outsmarting Siesta. They have no concept of right and wrong, nor does it bother them to be thought of as evil by others. They also completely lack empathy and is not beyond disposing of any SPES operants who are of no use to them. Though Hel appears female to Kimihiko and Siesta, they seem to identify as non-binary as evidenced by their masculine behavior, their use of the masculine pronoun "boku" and preference for masculine clothing.
As a villain in The Detective Is Already Dead, Hel appears to have little self-motivation for their actions apart from fulfilling their assigned role as a SPES operant. They follow the philosophy of a book known only as The Sacred Writ, which appears to be SPES's own version of the Holy Bible. When questioned by Siesta about their terrorist motivations, Hel only replies that they're only fulfilling their destiny as described in the Sacred Writ, implying SPES may be more cult-like than a pure terror-driven group.
The possibility of SPES being more cult-like is further validated when Hel reiterates to Siesta that they were born to destroy the world and are committed to fulfilling that role, much in the same way Siesta is committed to her role of saving the world. In this regard, Hel states they're both "the same." This, however, appears to be more evidence of cult-style brainwashing as Hel does not appear to have any original thoughts. That case is strengthened when Hel is defeated by Siesta and starts questioning their mission, the purpose of their birth, and if they really are as "significant" as they were made to believe by an authority figure known only as "Father."
While Hel is the first member of SPES to possess any level of authority in The Detective Is Already Dead, the narrative seems to also be presenting Hel as Siesta's intellectual equal and moral opposite. Like Moriarty and the Joker, Hel is easily capable of defeating Siesta but also appears to enjoy being challenged by Siesta. As the detective's moral opposite, Hel's lack of empathy challenges Siesta's self-righteousness, but it may also point to a lack of an actual identity as the real reason for that. With these developments in Episode 6, The Detective Is Already Dead appears to be setting Hel up as Siesta's archenemy.