This is a very good question! Since the 'master' is allegedly terrible at magic. Are mystic codes prepared by caster-type servants for their help prior to battle? It would make sense. Caster-type servants letting Ritsuka feel like a proper magi by making them mystic codes and such.
I keep saying "We're a dogshit
mage, but a good
person" for a reason; we're a D-rank Mage at absolute
best, the MC isn't even from a named family. Mystic Codes are basically loaded guns, the spells are pre-loaded into the garments like bullets and can be cast much more easily; we can cast Genderfluid and Gandr (which is basically Power Word: Taser) without one, and that's about it.
Wait... then can they not use mystic codes or am I forgetting? Is skills from mystic codes not count as spells?
Not to the same degree as being able to use them without the Mystic Code; It's not frowned upon but if you can't cast the spell without one, you can't really claim to be able to do so, and Ritsuka has the spirit and the willingness, but not really the ability. We can rally all the Heroic Spirits in the Throne because we can connect to them on a personal level, we don't need to be able to sling spells for it, and for that the Clock Tower will never forgive us.