WHEN ADDING SIDE - CHARACTERS...

When adding side characters, make them important! They might not be the main characters, but do not make them useless!

Make each character have a role in the story, or else, why add them to your story?

Whether they're a small part of a main characters development, or used to foreshadow a plot you're saving for later.

Otherwise, are they really necessary?
Nobody likes a character with unfulfilled potential.
Whats the point in a character that has no point in the story?

Make each character, even the smallest ones have a motive and a reason to be in the story!