Just remember guys, consent is absolutely important if you want to control another person's character in your post. Even for minor things, don't just assume they'll be cool with it. Ask first, either here or in a pm and if you're not okay with what someone else makes your character do, don't hesitate to say it even if it's me. Just like anyone else, I can make mistakes too sometimes so point it out if it happens.
As for the knight ranking thing, I was kinda just speaking hypothetically, but honestly
@deia876lat's idea isn't too bad. There's just a few things I'd like to add. For the knight apprentice, I think a blank shield would be best. The idea being to show that though they're training for knighthood, they haven't earned their knightly name yet so until that happens, they remain nameless in a sense. They'll mostly be found training in knight academies or under the direct tutelage of a full fledged knight, but they won't be officially knighted until they've passed a rigorous number of tests and a final evaluation of their skills through a duel with a knight.
Some soldiers however, such as my character Leo, can rise up from the ranks and earn knighthood by gaining enough merits through battle, allowing them to skip the apprentice rank entirely (with the king's approval). Unlike apprentices, such people didn't train specifically for the role of knighthood, but gained their skills and experiences through their time as soldiers. The difference between soldiers and knight apprentices being that most soldiers are just ordinary people serving in the military whereas knight apprentices specifically train to be ki users from an early age.
As for the other ranks higher than knight, no need to add an extra shield. Just have the sword(s) positioned behind the shield and there's your rank. Personally, I think blades crossed downward would look better for the knight captain. For the knight commander, it ought to be basically a combination of the senior and captain rank insignias, so crossed swords with one in the centre. I'll also say that the job of the knights isn't to command, that'd be the general's job, though they
can give orders to anyone of lower rank if they want. The most valuable asset about a knight is their fighting ability, so their only job is, well, to fight. As the most powerful ki users in the military, each knight is basically a one man army so they're only ever called upon to protect someone or something very important, or destroy a particularly dangerous individual or group. Think of them as special forces and artillery rolled up into one.
All of that is pretty much a long winded way of saying the knight commander wouldn't be in charge of commanding the army. Just the knights.