That is entirely up to Ritsuka, of course. But if I may be blunt, Miss Kishinami, her magical ability is much like yours; it was already mediocre to begin with.

[ Just looking at the facts!! ]

Should the laws of nature dictate that it is impossible—which I believe is most likely—I would advise that she contract with the most capable Servant she is able to support with her mana... [ So, prooooobably Robin? But he's gross and awful. ]