High school freshman Nao Misumi has known Sena Ikuta, his older sister Rio’s best friend, since he was in elementary school. As a self-proclaimed “sister’s boy”, Nao has always found Sena’s over-familiarity incredibly annoying. Yet, despite being loud, airheaded, and having zero sense of personal space, he could never truly bring himself to hate her. She has always looked out for him just as much as she has for Rio. She’s a total idiot, too carefree, and her sense of personal space is completely broken. To Nao, she’s supposed to be nothing more than a nuisance, and yet... “At first, I hated her for sure. She was noisy, and always took the things I cherished. So why is it that now—?” Even as he grumbles about her constantly calling him “Lil bro," he finds himself unable to leave her alone. How did this “annoying” fixture in his life become the one person he can’t look away from?