Shonen Onmyouji

Shonen Onmyouji is a series of novels by Mitsuru Yuki with illustrations by Sakura Asagi.  The novels revolve around Abe no Masahiro, the grandson of the legendary Abe no Seimei.  Masahiro is a young man who trying to surpass his grandfather and become the greatest onmyouji (or priest, give or take).  His partner/best friend is Guren, one of twelve spirits who serve Seimei.  Guren has the ability to take the form of a small mononoke (spirit) and in this form, Masahiro calls him "Mokkun."

I have seen the anime version of Shonen Onmyouji and I definitely recommend it.  It's a show where character development is just as important as action.  The relationship between Masahiro and Guren is probably one of the more interesting ones I've seen in anime. 

This series was one of those titles in limbo after Geneon stopped distribution last year.  However, with Funimation now distributing Geneon's titles for them, you should be able to find it on DVD in the U.S.