A sidekick is a buddy who comes with you on your adventures. If you’re a superhero, your sidekick helps you fight crime. Electra Woman’s sidekick was the teenage Dyna Girl, and in the ‘70s they defeated villains on TV.
A sidekick might be a work assistant, a best friend, or even a devoted dog that follows you to school every day. Sometimes this word signifies an imbalance of power, though; if you say, “This is my sidekick, Sarah," it implies that Sarah has less authority than you do. The origin of this word is unknown, but we do know that it appeared after the terms side-pal and side-partner fell out of fashion.