Luna is the name of the moon on Bear in the Big Blue House.
At the end of every Bear in the Big Blue House episode, Bear climbs to the attic to talk to the moon about the day's events. Bear and Luna sing "The Goodbye Song" to end each episode. More occasionally, the other characters sometimes also talk with Luna and/or sing "The Goodbye Song."
Bear and Luna are old friends. Luna often knows just what Bear's thinking and sometimes speaks in unison with him. Given that she's so old, Luna is full of wise advice.
Luna has a bit of an appearance change in the fourth season of the show. Her coloring is more yellow.