Maksis is Roen's true master. Even after his fall at the hands of Rem's father, Roen still remained loyal to him and began plotting ways to obtain the grimoire and restore him to power.
Roen served as Rem's pet dog but was in fact only getting closer to him in order to find out about the grimoire.
He gets closer to her to confirm that she is the grimoire, and threatens to kill her more than once. In episode 11, he claims that he "loves" her as well. The feelings are more of an insincere obsession since Ritsuka has the power of the grimoire and is the recognized daughter of his true master. In the game, Roen tells Ritsuka that Maksis loves Ritsuka and her mother and cared what happens Maria and Ritsuka
Roen and Marius are childhood friends. They know and trust each other for a long time but they lost contact because of the search for the grimoire. Marius tried to get the grimoire too and led through the storyline as the storyteller in Fortuna and the Stage Play Fermata and the contact to each other was not really mentioned. After the thing with Roen's Master, they are pretending like they don't like and don't even really know each other.