Masks
They all wore masks. Everyone has something to cover up, needs somewhere to hide.
Kanda's mask was a common one, aloof and angry; uncaring. He'd been wearing it for almost a decade. Ever since the... incident. Long enough that he'd started to forget things: how to make friends, how to be kind, how to care. They were all lost behind the mask, because every time he started to let it fall, tried to let someone in, it always went wrong.
Lenalee had arrived not long after he had built the wall. He had still been trying to perfect it. It had been weak, and he had started to let her in. After all, he was only a child at the time, and they had looked to each other for comfort. That was, until he had heard the yells in the foyer, the morning she was meant to come home from her mission. In that moment, as he watched them cart her away, a nine year old, beaten and bloody, barely alive at all, he sealed himself away once more. She still ran to him for comfort, but he no longer allowed himself to rely on her, or any human for that matter. They were too fragile.
Any time he started to forget this, or started to care, the memories would attack him. They reminded him of why the wall was there, and why he hid behind it.