If you are a Conjurer , i'd say Brelyna of the Mage college is a decent companion , or the priest of the Azura Shrine (forgot her name ) , she uses conjuration as well .
if you are a Destruction mage , i'd say J'zargo or Marcurio of Riften are very good in destruction
If you want a "tank" that uses magic , the guy at solitude's inn , he's a spellsword , he uses destruction magic and sword , so not a pure mage but still effective with magic and soak much of the damage
I played a mage to lvl 61, and agree with the above. I would add Onmund to the list, he is skilled in Heavy Armor, Destruction, and One Hand. (very similar to Balrand sp? the hire-able guy at the inn in solitude).
...enter the zone where they stand only at high level....
I use xbox 360, so can't do the console stuff... I have wondered, however, what exactly constitutes "entering the zone". Does this mean the city, or the tavern, or what?