Hiyoko, Kiiroi Ahiru ni Koi wo Suru

Ritsu Matsuoka is a bit of a natural airhead. When he discovers a small love letter on his classroom desk signed with an unknown address, he impulsively replies with a cheerful “I love you too.” This playful exchange of letters makes his otherwise boring classes a lot more exciting. However, his friend’s comment, “What if it’s a serious confession?” keeps nagging at him. One day, while returning to the classroom to retrieve something he forgot, Ritsu unexpectedly encounters Satsuki Hayato, a blonde-haired yankee known for his tough demeanor. As Ritsu sneaks a glance, he notices Satsuki blushing and writing something on the desk. Could it be? The mysterious author of the letters is none other than this intimidating yet intriguing younger boy?

(Source: Ichijinsha, translated)