First Love Monster, Vol. 1 — Akira Hiyoshimaru

When you find out who I really am...if you can still say you like me, then I'll be your boyfriend.
When fifteen-year-old Kaho Nikaidou leaves her sheltered home to start life anew in Tokyo, the last thing she expects is to nearly get hit by a truck! Saved in the nick of time by a handsome stranger, Kaho falls head over heels for him and boldly confesses her feelings. The handsome object of Kaho's affection agrees to go out with her, but her newfound bliss is short lived when it turns out that her new boyfriend...is a fifth grader?!