I'm planning to play through the game for the first time as a Mystic Knight. I just started, and I'm trying to decide what class I should choose for my main pawn.
Right now, it's a toss-up between Fighter and Mage. I definitely want a Mage in the party for healing, and having them as my main pawn would allow me to keep their inclination tuned for healing and support. On the other hand, I've read that a good Fighter can do wonders for crowd control, and again, having him as my main pawn would let me adjust his inclinations and skills instead of having to hunt down rentable pawns with the right setups.
Ranger was what I chose, when I ran a Mystic Knight. You'll find that your skills are slow, and your strategies are not easily set up; As a Ranger with skills like tenfold flurry, your Pawn becomes the interim damage dealer from a safe enough distance to avoid taking on serious injuries, distracting and drawing enemies (with good damage) while you set up your strategies. Another good class would probably be the fighter, if they have been taught to perfect block by watching the arisen. You basically want a class that will draw attention away from you while you prepare Sigils and cannons.
It sounds like you want a mage you can mold and trust, which is exactly what main pawns are all about. As someone that has hired hundreds of mages (I always want one in my party also), finding just the right one is very rare.
Also, you can set your mage pawn up to be more offensive, rather than supportive, because you'll be able to do the weapon enchanting yourself as a MK.
As a whole, it depends upon what strategy you are going for. I tended to make my pawns whatever I wished for in the long run rather than the immediate. So I have a varied stack of pawns on my myriad accounts, ranging from Fighters and Warriors to Mages and Sorcerers. I don't settle on multiple types of vocation for each character but a balanced one in HP, Stamina and the chosen stat they should be for whatever they shall be in my account.
For instance, I have a pawn I am building up as a powerful Sorcerer, but I have also taken some time to level her up as a Ranger for a time to get good strength and defensive stats. She'll still have over 740 Magic attack, but will also have a good physical defense rather than a weak one most magic-based pawns seem to have.
So, it really depends on what you wish for in your character's final stat count at level 200 (if you choose to actually get that far of course): many go for a 'pure' stat, which is to just level only as a certain vocation and nothing else, which results in very high stats for one particular area. Magic users have over 800 magic attack, but they will only have 180+ physical defense. I don't personally like that, so I try to balance some things out if possible. And in this game, anything is actually possible, except for a base stat over 1000! XD
So it's entirely up to you - but still feel free to listen to any of our advice!