From the beginning of the game Grim has confidently declared himself to be a talented mage that is going to become one of the greatest who has ever lived.
Grim is delighted when he is finally accepted as a student to the school, even showing an interest in joining the Arcane Response Unit.
Grim will often comment on his own amazing magical power, and his wish for Wish Upon a Star is to become a great mage.
He has Jamil perform a coffee divination for him and asks when he is going to succeed, but he drinks the coffee grounds so that there is nothing for Jamil to read. Cater observes, “But that’s poetic, right? It means your fortune’s impossible to tell!”
Grim refers to himself as “Master of Fire!”, and fire seems to be his go-to form of magic: he battles Ace and the mine monster with fire (after setting Kalim on fire during the school’s opening ceremonies), and puts his fire magic up for collateral with Azul in Book 3.
Grim struggles with using magic to arrange chairs in a vignette and Sebek observes, “We don’t have that level of control yet.”
Idia comments that Grim is “as lackluster with magic as he looks,” but after sensing the smell of what Grim associates with the black stones left behind by overblotters he becomes strong enough to blow an anti-magic door off its hinges.
Riddle insists that “Grim’s even more unskilled with magic than Ace or Deuce,” but the STYX technicians confirm what he did.
While “his test results for magic performance were all subpar compared to an average mage,” they discovered layers of ancient sorcery that have been cast on him and concluded that it is “likely some sort of curse—or perhaps a blessing—that triggers when certain criteria are met.”
(The color of Grim’s fire magic may be important. More here ->)
Twisted Wonderland Reference Note 