Ima tell you both. Lol here
It's connected to the childhood friend's background and why he ends up being orphan.
There is a tradition to choose a spirit user to conduct some ceremony once per few years. That years, the mc get chosen even though usually the one get chosen is girl from upper nobility. And the knight who becomes her partner/guardian is...our demon knight.
The plot more to mc learning manners, then there is some incident where spirit user disappear. From there the mc realized something about his childhood friend, and someone actually told about his secret.
Spoiler: His secret
When he still a little kid Reiss's (ML) family got killed because a woman who felt in love with his father. That woman is demon contractor. Then she force contract ML with demon. As how demon contractor need to pay something before he gained the power of demon, the woman chose so ML will fulfill the contract if he killed the person he really love. Basically, ML forgot about this for a while until some incident at his birthday, he remembered what the woman said...
And the next you know is when the mc heard ml said he hated her.
And we get to know that there is a reason why ml got chosen as guardian.
Spoiler: The Reason
The pope know that Reiss is a forced demon contractor. There is an item which can subdued the demonic power in him, but that item can only held by the guardian.
And the next they conduct the ceremony and tried to solve Reiss's problem.
Happily ever after~~
Its and okay novel and i wonder how the manga is going to turn out :o