69 (69 ろくじゅうきゅう) is a manga series by Masako Shimada. It was serialized in RuTile.
Satou Atsushi isn't neglected by his family so much as ignored completely. Left to fend for himself, Atsushi fails to learn human speech and instead learns "cat language" and "Dog language" from the family pets. When he's eventually socialized with other children, his speech is stunted and his social graces leave something to be desired. But on his first day of nursery school, he meets a gloomy blonde with the same last name who seems to be a safe enough choice in a first friend. After developing a rather sizeable fixation with Satou Manato, he finds out that there his first friend is seriously ill and has a twin; Shuuto.
Years later, two other friends begin school and find that there are two Satous in their class too. Two Satous who, at first glance, could easily be mistaken for twins, but there's a bit more to things than it first appears...