Dare ni mo Natsukanai Tobikyuu Tensai Youjo ga, Ore ni dake Amaete Kuru Riyuu

There’s an eight-year-old girl in my classroom. She’s a genius who skipped grades, with an appearance so adorable she’s like a little princess, but her personality is incredibly aloof, and she doesn’t connect with anyone. I thought the only person she’d open her heart to was her older sister at the same school… but one day, when I gave her some snacks, she suddenly got super attached to me!? This unexpectedly lonely genius princess starts acting incredibly sweet only toward me. Her affection knows no bounds, and she even tries to make me her stepbrother by getting me to marry her sister, the most beautiful girl in the academy—!? This is the story of a lonely genius little girl and an ordinary boy you could find anywhere. (Source: GA Bunko, translated)