Кейтер Даймонд

Dorm: 3rd Year Year Student of Heartslabyul.

He is a mood-maker who instantly lifts the atmosphere when the mood turns somber, and a paragon of communication skills. He sometimes refers to himself as "Kei-kun."

A heavy user of "Magicalam" (the social media platform in the story), he frequently snaps and posts photogenic moments and checks his feed after class. He is constantly on his smartphone—even while preparing for the "Unbirthday Party"—leading Deuce to label him a "terminal smartphone addict"; however, it appears that filming during preparations also serves the purpose of checking equipment and the state of affairs.

Alongside Trey—a fellow third-year in Heartslabyul—he often steps in to handle Riddle or support those around them. He also occasionally plays the "straight man" (especially during club activities).

Although he enjoys café-hopping, he surprisingly dislikes sweets. He also possesses a calculating nature; he is the type to skillfully use others to make things easier for himself—such as having underclassmen or the protagonist handle the task of painting the roses red. He can be ruthless at times.

While he generally maintains a smile and a lighthearted attitude, in desperate situations, his tone becomes rougher, and his way of addressing others shifts to be more aggressive. Unlike Trey, there are moments when he reveals little about himself.