Isidore explains how he met the incubus, Peter for the first time. Their story isn't touched as much in the main series, plus, when I was younger, I loved these two to bits that I thought they were worthy enough to have their own series that just focuses on them and how their relationship developed.
Isidore is a normal little angel, walking the streets among the kingdom, until he catches the site of a young demon. Not just a demon, but an Incubus. He may have just ignored the being, but he couldn't help himself upon seeing the pain and saddened look the boy gave, as soon as he looked his way.
Now living with such an impure being, many angels encourage Isidore to rid of him. But how can he? Yes, even he can't believe that a demon managed to be so dear to him.