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