Rem is Mikukier's son and heir. Mikukier is shown to feel no love for his son, seeing him as more of a servant. The two have a very abusive relationship, with Rem being terrified by simply being in his father's presence.
Mikukier has tasked Rem with the retrieval of the grimoire, because he needs it for his failing strength. He doesn't seem to be aware that Rem has chosen to forget his plans for the future, and instead make Ritsuka—the grimoire's human vessel—his new destiny.
Mikukier is the one who defeated the previous king and sealed him away.
It is assumed that they are enemies.