Shion Izumi comes into the manga with strong interest in Kei. He reveals to Kei that Gantz can be let known to the public through the internet site made by Nishi. Shion reveals knowing Kei was apart of Gantz . He brings up Kei was unharmed after an encounter with school thugs. Shion wants to see Kei's Gantz Suit. Kei has no time to respond when Shion drops the subject. Later Shion is among Kei inner circle of friends. Shion and others are amazed when a bizarre teen entered the class room covered in blood. He begins to kill as Kei had already left. Shion remains the last one standing and fights the teen to discover the Chibi Alien using a human shell. The alien stops toying around and knocks Shion out. Kei arrives to find Tae and the alien gives pursuit. Once Kei deals with the Chibi Alien Shion is sent to hospital to recover and all is calm for a good while.
Shion brings conflict to a period of peace. He is a previous Gantz Hunter that escaped the game and had his memory erased. Those memories had been returning and he wanted back in the game. Shion seems to be skilled in various areas. Everyone at his and Kei's school believes him a living wonder. Shion does whatever he pleases and even tried to take Kei's timid girlfriend Tae,away only to fail. Shion cleverly hides the fact he is a pyschopathic killer from all until the Shinjuku incident in Gantz. Shion also makes it a point to test Kei's nerve. He is secretly jealous that Kei is a hunter even though Shion knows he is more qualified. Shion while driven by Gantz's influence killed dozens in attempts to get back in the game. He was ultimately killed in a duel with Kei and left baffled upon dying as he felt he should have won in being the superior hunter. Shion is also responsible for the emergence of the new all star hunters Reika,Hiroto,Kaze,and Kenzo. Kaze attempted to overpower Shion but was shot in the face. Kenzo and Hiroto nearly defeat Shion with their telekinetic powers. He capitalizes on the pair as their powers have a slight gap in usage. Shion ultimately face Kei at a government tower. He shot Kei down and planned to commit suicide. Shion was hit by a dying Kei and killed. Shion returned to Gantz and was set only to rebuild his faded abilities in the upcoming mission. He is a borderline villain at first glance but is more of the antihero. Shion acts on his own most the time and cares little of the helpless new hunters on missions. The only time he joins with Kei and others is if he is overwhelmed by the enemy.Shion seems loyal to Gantz without question. He even goes so far as to kill Tae when she was able to take pictures of the team.
This is Shion's first mission in returning to Gantz's room. Shion is first to leave the room and takes off on his own.He faces off with a series of dinosaurs armed only with the Gantz Extension Sword. He displays great speed and skill more so than others that have worn the Power Suit. Despite his superior abilities once again Kei proved the better without his suit. Shion also seems to be a nemesis of the Vampires. This is likely due to events occuring in Shion's prior ventures as a hunter. The Vampires try to ambush Shion outside of Gantz but he is still in his suit. Shion easily defends against his oppressors as the transfer process takes him away from battle.
Shion makes Kei his best friend to ensure he doesnt tell the cops about the Shinjuku Massacre. Shion makes it a point to invite Kei and Tae on a double date with him and his trophy girlfriend Ryouko. The group go to Shibuya and Shion's past comes back to haunt him in full force. The Vampires target Shion as the only recent hunter that they know of. Kei is present but is spending his days like a typical lazy teen and decides to keep Tae and Ryouko safe. Shion like Kei wears his suit at all times now. Shion begins killing the vampires that attack by the dozens. He ends up killing them all to seal the deal with Kei as seeing him as more of threat than ally.
Shion acts alone once again on this mission. He doesnt do much since there are only three enemies to face.
Tae Kojima is a target of Gantz and Kei has left to protect her. Only Shion and Reika know of the girl's connection with Kei. Shion lets everyone know that Kei must be considered an enemy because of his relationship with Tae. He doesnt care if the target is human and is loyal only to Gantz. Reika and several others that trained with Kei outside of Gantz face off with Shion and a group of new rapists that like how Shion thinks. Shion goes off on his own to hunt for Kei and Tae. Hiroto and Kenzo stand in Shion's way. They are baffled as the X-Guns dont work on Shion. He uses the Extension Sword to cut down Kenzo. Shion learns that the powers of Hiroto and Kenzo bypass the defense properties of the Gantz Suit. Hiroto must tend to Kenzo and allows Shion to progress in his search about Tae's neighborhood. Two rapists come to Shion for back up as Kei killed one of them already. Kei and Shion duel in stealth mode until Reika comes. She distracts Shion long enough to get Tae from a cornered positon. Shion lets the rapists chase Kei all over town. He lies in wait until Tae eventually comes back to find and aid Kei. Shion strikes Tae down and she dies in Kei's arms. Shion hesistates to deliver a true killing blow for some reason. Tae is transported away by Gantz to end the mission. Shion receives 30 points for killing her because Tae had information that could expose Gantz to the world.