Five year old Jiyong meets a butterfly Six year old Seunghyun kills it

Seunghyun asks Jiyong to help him to buy an engagement ring.

Choi Seunghyun is a teacher of literature, every day he finds one letter at his door, he's the addressee, but there's no sender in the envelope. Every letter is telling a love story that makes him sad and lonely, then one night he meets a boy with pink hair and teary eyes and weak smile... The sender... Who's the sender?