The Prince in His Dark Days

The Prince in His Dark Days (王子様と灰色の日々, Ouji-sama to Haiiro no Hibi) is a manga by Hiko Yamanaka. It was serialized in the Aria manga magazine from April 2011 to May 2014. It is complete in four volumes. Kodansha Comics has licensed it for release in English.

Plot
A drunkard for a father, a household of poverty... For 17-year-old Atsuko, misfortune is all she knows and believes in. Until one day, a chance encounter with Itaru—the wealthy heir of a huge corporation—changes everything. The two look identical, uncannily so. When Itaru curiously goes missing, Atsuko is roped into being his stand-in. There, in his shoes, Atsuko must parade like a prince in a palace. She encounters many new experiences, but at what cost...?

Official
Official English site