Boukensha Guild ga Juunisai kara shika Hairenakatta node, Saba Yomimashita. The Comic

At just nine years old, Kiri leaves his sleepy village in the countryside and sets out to make a living for himself. With the help of a traveling merchant, Kiri reaches a bustling seaside town—where he's betrayed and almost sold into slavery. He manages to escape, but now he's all alone in the strange town with neither a roof over his head nor a coin in his pocket. The streets lead him to the Raging Kelpie's Tail, a guild of adventurers. Its colorful characters take Kiri under their wing and watch over the earnest boy as he overcomes challenges and grows both in height and spirit. (Source: J-Novel Club)