@Fireboy—
Japanese does not gender words in the same way as does English, creating challenges for translation. Partway into the series, it became clear from the illustrations that the creator of Suoh and of the body of Mute were a female, so that what had been rendered as “Master” should have been rendered as “Mistress”. There has been some speculation that Mute's mind is actually that of the creator; in that case, it would stand to reason that Mute would be a bitch. But the translator might just be giving a best guess in assigning a gender to Mute's pronoun.