Riddle says that he can sympathize with the prefect as, “I imagine it is difficult to make Grim heed your instructions. Heartslabyul has its fair share of problem students.”
Riddle encourages the prefect to go to him if they ever have their hands full with Grim, as he has “a lovely collar for him.”
Throughout Book 2 Grim seems to not trust Riddle, likely due to the drama of Book 1: he calls him “Ragin’ Riddle” is surprised when Riddle volunteers to help catch Ruggie (“What’s your angle?”) and says that he is “as square as ever” when Riddle leaves mid-investigation to make sure that no one at the dorm is breaking any rules in his absence.
Ruggie has a voice line about offering to babysit Grim for the prefect (for ten thaumarks an hour) and sharing grilled fish with them after Grim eats all their dinner.
The two meet for the first time in Book 2 when Ruggie uses his unique magic to force a trade for Grim’s sandwich (for Leona), which ultimately compromises Leona’s entire scheme to take down Malleus when Grim realizes what happened.
This experience may have contributed to Grim’s poor impression of Ruggie: when he learns that Ruggie buys food from the cafeteria to distributed to children in his neighborhood back home Grim reflects, “Never knew he had it in him,” and when Grim learns that Ruggie is refusing constant favors from Jack he says, “I woulda thought Ruggie’d be like, ‘sweet!’ if you offered to do stuff for him.”
Twisted Wonderland Reference Note 