Lancelot and Kay sound fine, actually. I was just using Sir Gawain as an example of how you could make the knights different from each other. The way you have it set up, it seems like it would take forever to actually get them, though. Correct me if I'm wrong, but it looks like you'd need to build (in order) a Peasant, Blacksmith, The Noble, Wizard Tower, Merlin, Sword in the Stone, King Arthur, and Round Table. I don't know about you, but my games don't normally last long enough to build all that stuff, not to mention all the resources it would take.
A friend of mine suggested another possibility for the Apprentice: "Protégé", which is someone under the teachings of a mentor. In this case the mentor is Merlin, so I think it fits quite well. Other synonyms include student, ward, pupil, tyro, and novice.