Tsuki has multiple personality disorder, and is often triggered by the moon's illumination.
When the moon is full, he's very nice, kind, and cheerful, when the moon is completely dark, he's evil and mean-spirited, and rude. When the moon is half-lit, he's a mixed combination of mean and nice (like Jekyll and Hyde) and rarely, if there's a lunar eclipse, he becomes bloodthirsty and tires to kill (and bleeds from the mouth as well)

Tsuki is a kanju, whose pupils and scalrae resemble the moon.