The Crybaby Boy's Vow (chapter) is the 9th overall chapter of Tegami Bachi Manga
Lag runs back into Sylvette's house, and promises her that he will find Gauche. Soon, he decides to shoot a shindan at Gauche's shindanjuu, Nocturne Op. 20, and immediately memories flooded back of Gauche being with Lag, fighting Gaichuu Jinro, letter deliveries, and eventually details about an anti-government organization named Reverse.
Gauche also asked Sylvette to give his gun to a boy named Lag Seeing that would probably come one day looking for him. The memories conclude into an emotional moment for both of them. Later, Lag is congratulated by the staff at the Bee Hive for becoming a Letter Bee. He then shared his dreams, including the same dream Gauche had, of becoming Head Bee.