Keido releases Segai from his services, giving him data containing information on Voids as promised, before going to the crystal tower. Meanwhile, Gai tells Shu to follow Yu, who helps Shu regain his memories. When he was younger, Shu and his older sister Mana Ouma, living on an island at Oshima, spotted Gai washed up on the beach. Gai spent the summer with them at their beach house, becoming best friends with Shu while falling in love with Mana. After experiencing the flashback, Shu finds himself in the crystal tower, where Keido has Inori bound. Keido explains that Inori was created to resurrect Mana, whose soul lies preserved thanks to the virus, in order to spread the virus to annihilate the human race, and he intends to create new race by marrying Mana. As Yu prepares a ceremony, another flashback reveals that Mana told Shu that she wanted to marry him. However, Gai later learned that she was infected with the Apocalypse Virus, which induced her insanity, but Mana threatened him not to tell Shu. Back in the present, Shu tries to stop the ceremony, only to be bound by crystal-like plants controlled by Mana. Gai and Ayase arrive in the tower to fight back, but Ayase's Endlave Steiner is destroyed in the process. The last flashback reveals that Gai asked Shu to meet him at a church to tell him the truth about Mana, but Mana came instead and tricked Gai into shooting himself with a defective gun. Shu was disturbed by this upon his arrival then rejected to marry her, which caused her to go berserk and spread the Apocalypse Virus all over Tokyo, ending up destroying herself from her own power out of fear. As a result, Gai vowed to become stronger, seeing as he was responsible for this, while Shu repressed his memories about Mana and Gai due to trauma. Shu, who finally remembers his past, extracts Gai's Void, a gun that forces out other people's Voids, and goes to rescue Inori while Gai heads to the pod to free Mana's soul. Keido tries to stop them, but Yu knocks him out for failing to marry. Gai reaches for the pod, but is pierced by the crystals protecting it. Shu has no choice but to stab Gai through his body with Inori's Void to destroy the pod. As the tower crumbles, Gai finally holds Mana in his arms and they disappear together, while Shu and Inori float to safety and mourn for his death.

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