Keizoku

They're both Buddy Cop dramas with male and female leads investigating weird, abnormal cases. Female leads are both weird and quirky and decided to become police officers to follow in their fathers foot steps. Male leads are both disillusioned with the criminal justice system. The writing and character dynamics are very similar.

Investigation/detective jdramas.
Goofy dynamic between the leads.
Interesting cases, often hard hitting.

Quirky characters, creepy cases, similar off-beat and dark atmosphere. Both also have male x female duo with great chemistry

Psychometrer Eiji is one of the dramas that Tsutsumi Yukihiko directed before Keizoku. Its also the drama that influenced the creation of Keizoku the most. Psychometrer Eiji is almost like a prototype for Keizoku and you can see the characterization and plot points that would be reused.

Investigation jdramas.
Set in departments that are looked down upon/essentially forgotten by the rest of the police.
— Aren't taken seriously, but are productive nonetheless.
Quirky chaotic female leads who annoy cold/rude male leads.
Both feature Ryu Raita in a supporting role.

Keizoku is apparently a parent story to SPEC though I see no direct correlation in story. SPEC leaned into supernatural abilities while Keizoku aimed to disprove and rationalize.

The shows have the same executive producer, Hiroki Ueda, and you can immediately tell when you watch unnatural that he was trying to recapture the same magic of Keizoku. The character dynamic between the leads in both shows have the same vibe and the over arching mysteries that connects the flow of the shows have the same structure. If you watch one you'll see the resemblance in the other.

Investigation jdramas.
Set in departments that are looked down upon/essentially forgotten by the rest of the police.
— Aren't taken seriously, but are productive nonetheless.
Both focus on a new character joining this environment (FL in K, ML in BSS).
Quirky chaotic and comedic in setting and character.