Series by 森つぶみ

Minato Usami, a high school girl, is thrilled when her father’s remarriage brings her a new stepbrother, expecting an adorable little boy. Instead, she meets Koshiro, a quirky and energetic elementary schooler who’s not quite what she imagined. Together, they navigate the ups and downs of their new family dynamic, blending Minato’s teenage life with Koshiro’s childish antics.