Ace of Diamond
was originally a researcher of BOARD's Humanity Foundation who graduated from the Sazahaibu University and was selected to be the user of Rider System 01, using the sealed "Category Ace of Diamond" (Change Stag) to become . From the beginning of the series, Sakuya begins to suffer a form of cellular breakdown, and only by sealing the Undead did the process slow down. Sakuya blames Karasuma for his condition, and the fact that Kazuma was appointed as his backup rider in case he could not cope on his own did not improve his relationship with Karasuma. When he attempted to seal the Locust Undead before it attacked BOARD, Sakuya is forced to abduct Karasuma for answers prior to him going into a coma. However, it is only revealed later that the "Karasuma" he safeguarded was only a decoy as Sakuya learns that it is actually his subconscious fear that is causing the breakdown.