During Spectral Soiree he says that the prefect taught him that “Even if an enemy’s too strong to face alone, we can still beat them if we work together” (in reference to the prologue) and shares this wisdom to calm a panicking Sebek.
At the end of Wish Upon a Star he says that he hopes the prefect will be able to use a wishing star for themselves next year, instead of Grim, and that their wish comes true.
After the events of Book 4 Deuce says, “I see you guys all the time, but it feels like it’s been years, for some reason.”
The prefect, Deuce and Grim pool their money together to buy a souvenir for Ace, and Deuce invites the prefect back to his hometown again after White Rabbit Fes (“I’ll try and be a better tour guide next time”).
In Deuce’s violent dream world of Book 7 he looks out for the prefect and Grim, saying, “You’re supposed to reach out to me or Ace when you head into non-Heartslabyul turf, remember?”
Deuce asks and receives permission from Dream-Riddle for the prefect and Grim to sit with Heartslabyul during lunch in order to protect them from hazing by other dorms.
When ordered to procure witnesses during a trial in Cater’s dream world the prefect and Grim are amongst the people whom Deuce selects (after Leona).
In Riddle’s dream Grim asks Deuce what his role will be in the final fight against Riddle and Deuce responds that, of course, his job is to protect the prefect.
Twisted Wonderland Reference Note 