@FireCamp im not justifying anything, its the implication of master and slave relationship is false.
this is a contract between a human and a monster, a familiar`s contract. in a way you can see it as her putting a curse on him. as she is free to take all his magic power to a certain limit. as long as he allows it or is unaware. im not saying that 'because she is a monster she is allowed to be a slave.'' its ''because she is a monster, she can make such contract.'
second is, this contract can be rejected by Mc, if he holds anything against her. it worked because he holds no malic towards her. even after all the shit she put him though.
also in fiction the word slave can be anything realy. in some a Slave is just a contract live in worker. heck there is a manga, that slaves are treated better than noble men.
those slaves just have to keep their master/lady company, fk her and punch people who cant retaliate. then get paid for it.