First Love Monster Vol. 4 (manga)
Though Kaho and Kanade break up due to Kanade's own immaturity, the fifth grader soon realizes just how precious Kaho is to him. Which leads to the completely mature actions of chasing her home, serenading her with piano-accompanied love songs, and even making an emergency marriage proposal?! Surely the age gap between the two means marriage is impossible--but Kaho and Kanade tentatively exchange vows! Meanwhile, it seems like a new threat is waiting to test the young couple's relationship--and that busty high school girl and devilish college student are at the center of it!