Koneko Toujou is that rare character who somehow manages to be both a fan favorite and a serious pivot point for the male lead's development in 'High School DxD'. She's introduced as this seemingly frail, quiet first-year student with a hidden past as a Nekoshou, a sub-species of the Nekomata. After the original Rias Peerage is defeated, she becomes the Rook of Issei Hyoudou's peerage, which is a huge deal for the series' power dynamics.
What I find most interesting isn't just her immense physical strength as a Rook, but how she represents a different kind of emotional anchor for Issei. Where Rias is the unattainable, regal queen and Akeno is the seductive, mature one, Koneko is the grounded, bluntly honest voice that often calls Issei out on his more ridiculous perverted antics, but from a place of genuine care. Her backstory with her sister Kuroka adds a layer of tragedy and a personal villain that Issei has to help her confront, which shifts her from a stoic side character to someone actively driving a plot thread. Her progression from calling him 'Senpai' with a cold distance to a more affectionate, if still tsundere, attachment shows a slow-burn character arc that's really satisfying to watch unfold across the light novels.