User blog comment:BrutalBear98/PG Removal: Spike (My Little Pony Friendship is Magic)/@comment-2059440-20191011121029

for most of the series Spike was a young child and he was thus not even meant to be "pure of heart" to begin with - most children are mischievious and prone to doing things they shouldn't, it isn't usually until they grow older that they develop any form of true morality beyond a natural sense of right versus wrong.. that's why Twilight etc exist, to help teach him about things.. like any kid.

as a dragon he's also always going to be more prone to being selfish and troublesome, he does his best to not be these things but MLP makes it fairly clear that very, very few characters are "pure of heart" as the whole point of the show is that friends as individuals are flawed but when they work together they are strong.. each character is meant to have some flaws that others help out etc etc..

so far the only one that is arguably Pure Good is Fluttershy - almost every other MLP character is certainly not Pure Good as they all have flaws and mischievious sides.. which is why they focus on working together in the first place.