This is what happened after 6 died. Akane was very distraught about hearing about the death of one of her closest friends, knowing that she'll miss his quirkiness, timidity, and his strange behavior.
That night, he visits her, and she thinks his spirit's come to stay with her, but instead of being happy, Akane is horrified to see how much he's changed. Even though she liked to see him smile, he's just not acting his strange self, his stripes are gone, but what bothers her most is that his key is missing. She asks why he would simply throw out the key that he loved so dearly, wouldn't let go of, was designed and practically born withm clutched on when he was scared, and even DIED with that key. After telling her that he's actually come to say goodbye to her and the world, she suddenly turns from sorrowful to enraged, asking him why he spent all that time on Earth when his oddities and friendship would add up to little to nothing in the end, and says that if his ghost is only leaving her behind, then he shouldn't have bothered to live at all.
She angrily yells at him to leave her and runs away, so ineffably enraged and grief stricken, her soul flies out of her body, even though she's still alive, and flies through the night, wailing and screeching in despair and agony, and the soul eventually discovers the body of 6. As soon as she reaches it, she wakes up, but the next day, she looks for his body, finds his key, and burns it all, hoping that now the key is gone, it'll join 6 and that the key will give him something to remember her, his family and friends, and the earth, all the while, partially whispering, partially crying, "Please take it... please take it... please take it".