Shiori Kishimoto is Kei Kishimoto's younger sister.  After Kei Kishimoto failed to get good grades, her mother stated she was worthless, and shifted all of her attention towards Shiori. This led to Kei's suicide. Nothing else is known about Shiori. 

Tokyo teens Kei and Masaru are killed by a subway train but awaken in a room with an ominous black orb that gives them weapons, suits and orders. Fighting bizarre alien monstrosities in a deadly game, will they win their freedom or die for the final time? 
(Source: Dark Horse)

Thought your life was bad? Sometimes, death is worse. There is no salvation, peace, nor god waiting to receive you into their care. But wait, a god? Maybe you are talking about that big black ball stuck in the room with you. Now you are thrown into a game, fighting green aliens and robot monsters for the chance to survive. 
When Kei Kurono is killed, he thus finds himself caught in such a game; a test of his skills, morals and will to survive. His life is not his own, his death spat and trampled upon over and over again. What happens if he does not listen? God knows. 
A word of warning: Gantz is not for the faint-hearted, but neither is it as simple as it looks. Gore, rape and violence is rampant, as are portrayals of greed, violence, and all the ugliness that one sees in society today. 
[Written by MAL Rewrite]

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Shiroi Kishimoto