Kaiba, Tokaku Azuma's former instructor at the assassin-training Private Academy 17, wields authority through cryptic text message riddles sent for undisclosed reasons. This adult male presents with distinctive white hair typically styled at neck length, his eyes often obscured. He orchestrated Tokaku's transfer to Myōjō Academy's Class Black, assigning her to assassinate Haru Ichinose. During their final meeting at the academy, Kaiba deliberately provoked Tokaku, revealing his past praise of her abilities was insincere. He instead declared her existence a "failure of nature," citing her inability to kill as the reason for her selection—framing her mission as a test of survival: kill or be killed. This highlights his manipulative mentorship. Throughout the series, Kaiba persists in sending Tokaku riddles, ostensibly to hone her assassin's analytical skills; one such riddle involves interpreting the phrase "the world is..." Their relationship remains defined by psychological tension: he dismisses Tokaku's growing moral conflict over killing Haru, yet simultaneously acknowledges her potential to overcome perceived weaknesses linked to her Azuma clan heritage.