Jijou o Shiranai Tenkousei ga Guigui Kuru.

Synopsis

From Square Enix: One lonely, gloomy fifth-grade girl is the target of her classmates' relentless bullying and teasing - that is until a new kid arrives on the scene. Friendly Takada is as clueless as he is well-meaning, but somehow he possesses the magic ability to start drawing "Grim Reaper" Nishimura out of her shell. As the elementary schoolers experience all the fun of a childhood summer together - from going to the pool to picking sunflowers to watching fireworks - unusual friendship blossoms.

Synopsis & details from MangaUpdates