Silvio's closest friend would have to be Avril (she doesn't have a a profile or even a finished design yet,,,). She's Alain's younger sister and the person Silvio's father wanted him to marry. Avril does have feelings for Silvio, but he has come out to her as gay, so she's just happy to be this close to him.
As prince, Silvio doesn't have many friends or people his age to be with, so Avril has become him emotional support throughout his story. Silvio would describe Avril as kind and caring but incredibly stubborn. Since Silvio tends to not handle his emotions very well, she is very hard on him at times. Silvio also likes how positive and optimistic Avril is, which helps him fell better.
Avril was the first person outside his family that Silvio came out to, so she means a lot to her, especially when he has no one else to rely on.