Once upon a time, the sidekick was an unsung hero (or villian). Just a tacked-on entity that occassionally popped up to help. Nowadays, the sidekick enjoys many perks, including individual fanbases, promotions to full fledged heroes, and even their own syndicated TV shows. They've come into their own.
But writing a sidekick is no walk in the park. They need to embody the essence of their mentor while at the same time giving them support. They are their own person but pursuing a similar goal as their mentor, for their own individual reasons. Often this causes a clash with said mentor. That can be challenging to write. You need friction and conflict, but at the end of the day they still need to be able to reconcile.
Chances are if you write a hero at some point you'll have the difficult duty of writing their sidekick. This week, pick a sidekick and explore their role in the story.