There was some kind of incurable disease that people have known of. For some reason, 'Alter Ego' can cause identity disorders, which shows in sudden mood changes, where a person can be someone they haven't thought they could be. In some cases, an 'Alter Ego' can be your protector, but also something that destroys your life. You have noticed some minor behavioral changes in your friend, but you never thought that Natsume had such a thing.

"It was not me who did that! It was ME!"