When Kantarou was a kid, he was fascinated with the legend of Onikui (= "oni-eater"), a strong youkai who had been sealed - a black-winged tengu to be more precise. So he decided to become strong in order to free him. See, they were so destined to be together. ;-) A few years later, Kantarou managed to free Onikui and called him Haruka. When you name a youkai, this youkai has to obey you. ^^ Youko is a fox demon who was also named by Kantarou, but don't worry, she's very cool, there's no het romance here.
The series has a lot of wonderful Haruka/Kantarou moments. Not that many in the filler episodes, but classy bad guy Raikou is very good at inducing Harukan!plot. ^_^
To come back to the het, there is indeed a het couple. Sugino is a white-winged tengu and he is married (?) to a youkai called Muu-chan. And yes, Muu-chan is a girl, a girl who only says "Muu," but if Sugino says it's a girl, it must be a girl. Here comes the other icon I got today. This is Sugino with Muu-chan. Please don't faint. XD
And to finish, let's squee about the seiyuu. ^_~ Sugino's seiyuu is the famous Hoshi Soichirou (Goku in Saiyuki, Gakuto in Prince of Tennis, Camus in Meine Liebe, Sunao/Ran in Sukisho...). Haruka's seiyuu is Takahiro Sakurai (Kira in Bleach, Jenos in Black Cat, Orpherus in Meine Liebe, Yuri in Kyou Kara Maou...). Kantarou's seiyuu is Kouki Miyata (Hanatarou in Bleach, Murata in Kyou Kara Maou...). All of that is quite nice to the ears. :-)