The predecessor is seen in Some time ago... where he is caught by Mistress teaching himself some "proper magic" without her approval, leading to him being tortured and killed. Mistress then has her guards put up the sign advertising for the position, which Ivan sees.
While Ivan was kidnapped in the Glove/Emancipation story, Mistress hired a new boy, but she was unhappy with him and gave Ivan the position again.
Following the events of Ivan screwing up big time in the story arc about the arrow of regret, the monster, and the immaculate city, Ivan is shot with the arrow of regret and the sign is put up again, but it is unknown whether a successor has been hired. Ivan was then found to be still alive but in a magical sleep, so it is possible that Mistress has decided to hold the position for his eventual return, instead.