Haruaki Yachi is a high school student living alone in a home that gradually neutralizes curses. He possesses unique immunity to curses affecting owners of cursed items, stemming from both his environment and innate constitution. His archaeologist father, frequently overseas, sends him cursed artifacts to remove their curses, which fostered Haruaki's strong domestic independence and cooking skills. His surname combines the kanji for "night" and "knowledge," reflecting his family's background.

Initially responsible yet occasionally indecisive, Haruaki avoids conflict but intervenes decisively to protect others, especially friends facing danger. This trait emerges when he encounters Fear Kubrick, a cursed torture device in human form, and vows to help lift her curse despite risks from organizations seeking to capture or destroy her. His combat abilities are limited without assistance; he wields Konoha Muramasa, a cursed sword and childhood friend, utilizing her "Killing Counter" technique to target opponents' weapon weaknesses. Later, he temporarily employs other cursed tools like the poison-injecting «Alius», the «Returning Kukri of Childbirth», and the sword Kotetsu during critical battles, reverting to reliance on Konoha after initial training from mentor Lilyhowell.

His relationships drive key developments: he anchors Fear’s emotional stability as she confronts her violent past; maintains a protective, sibling-like bond with Konoha; and accepts Kirika Ueno’s cursed tools without judgment, strengthening their trust amid her struggles. Throughout various arcs—including conflicts with antagonists like Peavey, the Sovereignty doll, and the Alice and Vivolio families—he evolves from a passive recipient of cursed objects to an active defender, balancing his aversion to violence with tactical resolve when allies are threatened. His home becomes a sanctuary for multiple cursed beings, cementing his role as their emotional and tactical center.