After finally adopting a child, the Sonoyamas were unprepared for the mystery that would soon unravel. Hitori seems to be the perfect son for his loving parents. He's handsome, intelligent, and grateful for his new lavish life—but he has a dark secret. Hitori is actually the twins Migi and Dali pretending to be one boy. And they have a terrifying motive behind their hidden identity. 
 
(Source: Crunchyroll)

Under the table is the angel's secret.
 
The boy's name was Hitori. A stork brought the angel into the lives of a middle-aged couple who were not blessed with children. His parents were kind, his house spacious, and his meals hot. But to protect the happiness he had received, Hitori continued to hide an important secret from his parents...
 
(Source: Anime News Network)
 
Note: Includes 3 extra chapters.

Osamu Sonoyama