Declan's fascination with miracles began when he was rescued miraculoulsy from an avalanche.  He's made it his mission to investigate as many miracles as possible in a quest to explain what science cannot.  His faith and openness to the existence of miracles is tempered by the skepticism of his sidekick.
 

Peggy's cynicism began the day her husband died.  She lost her balance as well as her faith, and doesn't seem to know how to get it back.  She's very disillusioned and angry, and is having a great deal of trouble letting go.  She continues to investigate these miraculous phenomena with Declan, hoping to prove once and for all that miracles don't exist, but she may finally begin to heal her broken spirit and her heart as some of their cases force her to face her demons and put them to rest.
 

Miranda is a bit of an enigma.  She's extremely intelligent, though very introverted
and doesn't seem to like Peggy very much at all.  She functions as Della to Declan's Perry Mason and he wouldn't be able to do any investigating without her.  More will no doubt be revealed as the show progresses.

The cast list would not be complete without the mention of Mole, so named for the mole on his belly.  Declan acquired him in New Mexico, and he has proven to be a good companion, and even managed to help Declan with his investigations on occasion.