|Mutsumi Kai睦実 介|
|Hair||Grey, Shoulder-length, V Bangs|
|Body||Pale, Slim, Young-adult|
|Clothes||Blouse, Coat, Gloves, Knee-high Boots, Military Boots, Ribbon Hair Tie, School Uniform, String Ribbon Tie, Trousers|
|Personality||Kind, Loner, Taciturn|
|Visual novels||Main character - Jack Jeanne|
|Voiced by||Kasama Jun|
Notable even among other Jacks, Kai is a Jack Ace; a Jack that plays main roles. He is in a pair with Takashina Sarafumi and Kai acts as a "vase" to support the "flower" of the stage, Sarafumi. He's taciturn and likes his alone time but he's always watching over his Quartz class and his kindness can be seen in how he helps fellow students in need. It seems that Kai's always thinking about what he can do as a Jack Ace.