Now that the two of them are back in the forest, we find out that Yu and Shuichi were once friends, and that Yu's sister, elena, and her friend, kaito, were also friends. But, when elena's friend, ho, died, she was replaced by a monster, and now the monster is growing larger and larger. Yu says that he's going to forget everything, including his sister's sorrow and her pain, because he doesn't want to be a part of this horrible thing. He's also going to be there, smiling, like an idiot. Yu tells us that when they merged, he saw his erased memories, which means that there's a lot of information about his sister hiding in his mind. This is why they're going to try to get inside the monster's lair, because they don't know what it is yet.