Kyon is an infamous serial killer. He is some sort of ally to the Scarf Heroes, even though he loves killing. He has a twisted sense of justice, killing for the sake of hope, and for the sake of despair. He suffers from dissociative identity disorder. He has two other personalities he calls Nice and Mean. Nice kills loves hope, and Mean loves despair.
Nice and Mean
Kyon has somehow managed to live with Nice and Mean his entire life. When Nice takes over, he attacks anyone who's not 100% "good". Mean tries to "punish" Kyon for some reason, by cutting his arms and legs. One day, Mean managed to remove Kyon's entire right arm, which led to his robotic arm.
The Observer is Kyon's weapon. It's a robotic arm he created after Mean chopped his arm off. It has many abilities, but since it's an arm Kyon created himself, the only things that work are the knife fingers on the end of the arm. He uses them to kill.