Starlight

Starlight is an animatronic who became sentient in Moon and Sun's old body  from the left over code that was in it. Starlight considers Moon as his older brother and feels more comfortable around him. After awhile Starlight was redesigned so he could look unique and so Sun and Moon could tell him apart from one another. Sun and Moon are slightly taller then Starlight as their old body was slightly smaller. Starlight is just one person but he has both a day and night form. Unlike with Sun and Moon, it doesn't hurt him when he switches from day appearance to night appearance.