Build a unique plot around the outsider's POV. Maybe the outsider witnesses something strange about Danny that makes them curious. For instance, if it's a teacher, they might see Danny disappear right before a ghost attack in school. This creates a mystery for the outsider to solve, which drives the story forward.
First, choose an interesting outsider. It could be a new character, a minor character from the show like the lunch lady, or someone like Valerie who has her own view of Danny. Then, research the character's personality and motives. If it's Valerie, she's initially out to get ghosts, so her view of Danny would be colored by that.