Mana Ouma is Shu's older sister. She has a mysterious connection with Gai, Inori, and Shuichiro. She is the first person to come in contact with the Apocalypse Virus. As Gai says, someone who is infected by the virus becomes crazy. Mana turns from a sweet loving sister into a sadistic woman who tries to marry Shu and mix their genes together to create a new world. As Shu pushed her away, she started an uproar that caused Lost Christmas to occur on December 24th, 2029. Within the outbreak Mana killed herself as well. In episode 12, Inori was the reincarnated body of Mana and used as a sacrifice to be rebirthed. Her rebirth was prevented and died along with Gai by Shu.
Mana has long, whitish pink hair with a white flower pin on the side. She has orange/red eyes. She wears a light pink dress that matches her hair. At times in the flashbacks she wears a white uniform and a pink winter coat. She holds a strong resemblance to Inori, her reincarnated body and is very beautiful. Her hair is parted in the middle and the front strands of her hair are tied.
At first, Mana is portrayed as a sweet girl, gentle, kind, and a loving sister to Shu. When she became infected with the virus , she became sadistic that she also even tried to have Triton kill himself.
When infected she wanted to tie the knot with Shu and mixed their genes to make a new world together only to have Shu push her away and caused an uproar in December 24th 2029 "The Lost Christmas" incident.
Along with Shu, she discovered the body of a young boy washed up shore on the beach, unconcious and with bruises covering his body. Trying to recover the young boy, she gives him CPR and the body awakens. Because he was found at sea, Mana happily tells him that she will call him Triton. The young boy is later revealed to be Gai. The three of them: Shu, Gai, and Mana spend the happiest summer of their lives before their happiness ended on Lost Christmas.
One day when Mana was having a conversation with Shu, she asks him what he would do if she were to get married. She even says that she might even marry Triton but eventually says that she is joking and wants to marry Shu, kissing him. The very young Shu simply stares at his sister, not knowing how to react.
Another scene shows Triton, the young Gai confront Mana and tells her that those who are infected with the virus become crazy and different, noticing the changes in Mana's personality. Mana lifts her head to reveal the purple crystals forming on the side of her neck, showing that she indeed is the victim of the virus.
Mana was the first person to get infected with the Apocalypse Virus. Inori was later used as a sacrifice, so her soul would enter Inori's body to cause another uproar worldwide with Shuichiro. Before getting infected she displayed a kind and gentle side to Shu and Triton.
On Christmas Day, Triton decides to reveal to Shu about Mana's condition at the Church of Roppongi but Mana shows herself first instead of Shu. She gives Triton a box, telling him that it his Christmas present. He opens it to see a black gun and a few bullets. Mana tells him that he is her "knight" and asks him to protect her, pointing to the golden star sitting on top of the Christmas tree in the church. Having fallen in love with Mana for a long time, Triton (Gai) points the gun and fires. It is later shown that Mana had tricked him into shooting himself as the bullet backlashes and hits him in the stomach. Shu arrives at the church not soon after and sees the unconcious and bleeding Triton on the floor. Mana, in a frightening state tells him that she wants to marry him. After Shu pushed her away and calling her a monster, she ended up destroying herself with her uncontrollable power and asked them for help in that day.
At the end of episode 21 we see Mana being reborn by sacraficing Inori. First Gai needed to erase Inori's memories for Mana to be reborn. Now she will have to choose her Adam, either Shu or Gai.