Cater’s unique magic Split Card (hidden meaning: the Fluttering Hand (of cards)) enables him to make duplicates of himself, which seems to use up a lot of his magic.
In Book 7 we receive an explanation of Cater’s unique magic from a fake-Cater of Riddle’s dream, meaning the information came from Riddle, not from Cater himself.
Real-life Trey is also present and agrees with the explanation, but as Cater himself was not there to verify we cannot be sure how accurate this information is (as of this writing):
Dream-Cater: My clones’ attack spells don’t pack anywhere near the oomph that mine do. And the real Cay-Cay’s magic gets a major downgrade while I’m controlling my clones. Though I guess that only makes sense.
Trey: Yeah, it’s basically casting multiple separate spells simultaneously.
Dream-Cater: Exactly. You could think of it like cutting a cake into smaller pieces.
My max reserves stay the same, so splitting them across multiple clones means a big drop in individual power.
Cater uses his unique magic in Book 1 both to paint roses and to beat Ace, Deuce, Grim and the prefect in a fight, in Book 2 to create decoys, during Halloween and during Playfulland, where Kalim says he once use the ability to liven up the pop music club’s first live show, which they played to an otherwise empty house.
Cater also uses his unique magic in Book 7: in his own dream, in Ace’s dream and against the darkness summoned by Malleus in the finale.
He also uses it during the opening ceremonies in order to paint roses white, and in a vignette to avoid having to do laps during gym class.
※ Note: The incantation of Cater’s unique magic is written in two different ways on EN, but is always the same in the original game. More here ▶︎
Twisted Wonderland Reference Note 