Yu, Ga On
He's a ghost looking like a high schooler. He has almost no memory of his previous life (remembers only his own name), doesn't know how he died, when did it happen, who was his family etc. Usually he's friendly and cheerful, especially around Haram, but when left alone, he tends to drown in his thoughts. Actually, he's not a ghost, but a fragment of memory, and he did not die—his "real" self just wiped off some part of his memory because of a tragic event. That's when Gaon The Ghost was "born" and when the actual Gaon started to have partial amnesia, which changed a bit his personality, too.