THE PERFECT INSIDER

The story of the original Subete ga F ni Naru novel revolves around Souhei Saikawa, a member of the Saikawa Research Lab. He goes on a vacation held by the lab, and Moe Nishinosono, the daughter of his mentor, joins the group on their vacation despite not being a part of the lab. There, the two end up finding a corpse. The two work together to solve the mysteries of what becomes a serial murder case. (Source: Anime News Network) — Description from AniList community (anilist.co).