Ni-kai no Youkai Oujo (Volume)

is the first volume of the Ni-kai no Youkai Oujo light novel series, written by Ryuji Arai and Illustrated by Kantaka. It was released under the Dengeki Bunko Imprint in May 2005.

Synopsis
"You inherited a mansion left by a distant ancestor." Yukito, a lonely high school student who came to a town near Tokyo, was guided by a sudden letter. However, in the old samurai residence, the Hyakki clan, a youkai who longs for Yukito's ancestor Onmyoji as a benefactor, settled in. The beautiful girl, Youhime, who unites the Hyakuki clan, unilaterally recognizes the upset Yukito as the main, and begins her cohabitation with her clan party. Yukito keeps being swayed by youkai who are good but out of common sense. Meanwhile, at that time, the evil youkai sealed by Yukito's ancestors are also released. The comedy series opens.

(source: Amazon JP - Translated)