Mary Lennox (The secret garden, F.H. Burnett)
I'm interested about this one
also the other characters of secret garden
Medlock? - i think Medlock is ISTJ
For Mary Lennox i agree with you - i also think she is an ISFP - also type 4 in enneagram. why type 4? Cuz once she said if everybody would tell her smth she will do she would never do it to spite them. She is a bit arrogant and preserving of her own identity - type 4 therefore. At least in my opinion.
Piggy from The Lord of the Flies probably was ISTJ. You'll be able to find lots of essays analysing him because it's a classic often studied in schools.
Piggy was INTP. He represented the role of the intellectual in society. He knew the right thing to do, but he needed to establish a rapport with a leader-type intelligent enough to understand the value of his contributions in order to have any effect. Nobody listened to him on his own.