|“||It'd be smart to forget all about it.||”|
–A Police Detective, almost lampshading Kouta's suspicions of the KPD
The Kamakura Police Department and its officers play a role mostly in the early part of the Elfen Lied series, but their depiction tells us much about the essential nature of the fictional universe.
In the series
The first two detectives shown are in pursuit of the recently-escaped Lucy. They appear to know who and what she is, raising the question of how many are either, acutely aware of the goings-on at the Diclonius Research Institute, or are else directly employed by the Kakuzawas. In fact, the level of cover-up and silence regarding Diclonius births can suggest that the Kamakura PD worked hand-in-hand with the Kakuzawas. On the other hand, Chief Kakuzawa had clearance from the very highest levels of the Japanese government, meaning that the police would have no choice in obeying his wishes.
The two main detectives shown (in both the manga and the anime) are first shown questioning Kouta about the girl he called Nyu, of whom he pretended to have no knowledge. During their exchange, they found Kouta evasive, and Kouta saw that they changed their story from one about an escaped mass killer to a lost girl. The next two detectives Kouta met (in the anime, they were the same two; the manga leaves this unclear) was right after he and Yuka (in the anime, just him) were attacked by members of the Special Assault Team sent out to retrieve or kill Lucy. While in the hospital, police told Kouta he that perhaps the attackers were military cosplayers or maybe a biker gang, and they not so subtly urged him to forget the whole incident. When Kouta met Mayu, who had been on the beach that same night, she confirmed that there were soldiers present. The confirmation that the police had lied soured Kouta on contacting them under any circumstance, fearing, rightfully, that they would take away Nyu and later Nana. No further interaction between the residents of Maple House and the KPD occurs in series; whether they saw through Kouta's evasiveness or if it marked him for scrutiny is also unknown. Late in the anime, the same two detectives questioned the Crepes' merchant once encountered by Nana, who was shown her photo and confirmed via nodding that he had seen her.
It is unknown what level of police connivance existed during the military raid on Maple House, but such a massive presence could not have gone unnoticed. Oddly, despite talk that the residents had harbored a dangerous fugitive, no discussion of charges was ever discussed. It appears possible that the police aided in clearing the beach of blood and bodies after Lucy killed the Unknown Man and split Bando in two during their climactic battle. The police were part of the force that kept residents from witnessing the expedition to capture Lucy using Mariko. In a late anime episode, Kouta and Nyu find one side of the police cordon members all unconscious, though it remains unexplained how they got that way.
Also unknown is whether members of the Kamakura PD also served as guards at the Diclonius Research Institute, though some of those guards having military or police backgrounds seem likely. Arakawa, upon finding the remains of her boss, directly urged Kouta not to call the police, so perhaps not every member of the force was in the know about the Diclonius. If any members of the PD were too deeply in with the Kakuzawas, and especially their secret agenda, it seems likely the Japanese government would move to purge and if needed, imprison them to keep its involvement a secret. In all likelihood, the end of the Diclonius War saw the KPD revert to traditional police work.