Jigoku Kurayami Hana mo Naki

A beautiful boy detective and his NEET assistant's "hell-bound" case file! Seiji Toono, who sees criminals as monsters, meets a handsome boy, Shiroshi Saijou, who runs a consultation office in a weathered Western-style house. For some reason, he ends up working as an assistant for Shiroshi, who has excellent insight and bewitchingly good looks... (Source: Kadokawa, translated)