Cater name-drops Trey fairly often, sharing Trey’s homemade sweets with Ace, Lilia and Kalim throughout different vignettes, and attempting to keep tabs on Trey during the welcoming party for new Heartslabyul students.
It is Cater who escorts Trey to the nurse’s office when Trey catches a cold (after going on an errand for Riddle in the rain), and the two collaborate to hide unpainted roses from Riddle in Trey’s ceremonial robes vignette, working behind Riddle’s back to keep peace in the dorm.
They also go “halfsies” on a teacup and saucer set for Riddle during Firelit Sky, and Cater teases Trey about Rook’s nickname for him of “Chevalier des Roses”.
Trey says “Cater, I’m going to get cross with you”, but we have never seen either get legitimately angry at the other. Riddle coerces Cater into baking a cake during Wish Upon a Star to ease Trey of some of his responsibilities, but it does not go well.
Trey goes to Cater’s rescue despite Riddle’s protests, ultimately turning the situation into an opportunity to grant his own Starsending wish.
Halloween is a stressful time for Trey: when it is revealed that the majority of Heartslabyul has disappeared Ace asks where Cater has gone.
Trey responds with “….”, Riddle explains that Cater cannot be found and Ace reacts with “Oh geez…” (No spoilers, but Cater is again a source of stress for Trey in Vargas Camp 2)
It is Trey who identifies the missing Cater during Halloween, saying “On the outside, at least, Cater can come off as flippant and fickle…but he would never take out his anger on someone else”. Cater apologizes profusely afterwards, with Trey assuring him, “Don’t worry about it. No one blames you.”
It is thanks to Cater that Trey puts his unique magic on display during Book 1 of the main story, but during Riddle’s overblot it is revealed that Cater had had no idea of the true extent of Trey’s abilities.
Trey is not the only one keeping secrets between them, however: Cater had been hiding his dislike of sweet foods from Trey, but Trey was able to figure out the truth on his own. It seems he cannot, however, deduce what it is Cater’s true wishes really are, or why Cater refuses to confide them.
A lot of information about Trey’s character comes from Cater, such as the vignette where Cater reveals that Trey is slow to anger, “but when he DOES get bent out of shape, he can stay that way for a while”.
Prior to the start of Beanfest Cater warns his fellow teammates Deuce and Riddle that “it’s the guys who act all harmless that you that you really gotta watch out for!”, in reference to Trey. Riddle gets captured by Trey the next day and says, “Cater was right all along.”
Cater and Trey are equally responsible for Riddle’s explosion at Ace for bringing a chestnut tart to the unbirthday party in Book 1, but despite Cater asking “Are you really okay with this” in response to Riddle’s rages, Trey responds “What do you expect me to do about it?”, and ultimately neither of them take action until Riddle overblots.
Cater does go after Trey again for “giving him a pass” for Riddle’s foul mood, asking if trey has “pampered him like that” since they were kids. Cater says, “I haven’t been around you guys that long, but doesn’t it ever get…exhausting?”, but Trey insists that Cater would be the same if Cater had ever met Riddle’s mother.
(On EN Trey hasn’t met Riddle’s mother, either: his history with Riddle was rewritten in translation.)
In Book 2 Riddle gives Cater permission to leave rather than stand and face an over-blotting Leona. Cater responds with “We do this together, housewarden!”, but his reasoning is “Trey wouldn’t let me hear the end of it.”
Cater is very much aware of the bond between the other two, and in a chat he says that Riddle and Trey are, as far as Heartslabyul’s concerned, like the Queen and King of Hearts.
Twisted Wonderland Reference Note 