Steady Steps Towards Family

A few years after Fujimoto Shirou met Mephisto Pheles, a rather strange and calculating boy, whom he made a bet ( and lost ) with, he is faced with the consequences of his actions when the boy has returned. And he has not returned alone, bringing with him his little brother; a little boy named Amaimon.

Shirou is now placed with the responsibility of raising a boy who has about as many mental and physical issues as the twins he's had in his care for years.