A harsh and perfectionist mentor, who teaches advanced shaping and "defense against magical attacks of all sorts" for the fourth year. Xvim is Zorian's mentor at school. Zorian isn't happy about it; he didn't get to choose his mentor because Ilsa didn't get to him in time.
He likes to throw marbles at people and tell them to go faster. He is deliberately harsh to his students and impossible to please for at least their first month, to "build character and weed out the unfit" and to humble them. Naturally, the time loop, being limited to a month's time, meant that Zorian never experienced Xvim's true teaching until Xvim became suspicious of Zorian's impossibly advanced shaping skills, and Zorian informed him about the loop. Since that time, Zorian has found him to be a demanding but very effective teacher.
Xvim actually does consider shaping exercises to be important, as he explains to Zorian :
"What did you think shaping exercises were, if not unstructured magical abilities ? Do enough of related ones over the years, and they're bound to build up to something greater than the sum of its parts."
Zorian considers Xvim an archmage (that is, a mage who has mastered several fields of magic to such an extent they could outdo a specialist mage in those fields).
Xvim prefers to use defensive magic in combat. During the siege on the summoning he uses defensive abilities almost exclusively. He repeatedly defends his fellow mages. When Quatach-Ichl fires an annihilation beam towards the battlegroup, Xvim conjures a green shield that pins the annihilation beam to it. He uses purple clouds and globes of ectoplasm to absorb myriad enemy spells.
During the same battle, after he has run out of mana, Xvim jumps in front of a deadly attack aimed at Zorian, giving him enough time to read the summoning mage's mind and learn about the conspiracy to summon Panaxeth.