Allies, Captains, Sidekicks
Allies, Captains, and Sidekicks are inherently intertwined mechanics. They use labels found on cards in a very similar fashion to Secrety Identity.
Dr. Doom here is both an Ally and a Captain, as seen in the upper right hand of his card
Valeria is a Sidekick, again because her card has the Sidekick label
Captains will have traits that normally affect only Sidekicks. Some times allowing them to use standard or special powers.
Allies will have traits that affect Captains and Sidekicks. So far, these traits are all SIDELINE ACTIVE, which means you don't have to play an Ally on your starting force to gain the use of the trait. However, if you do play them on your starting force, you still get the benefit. SIDELINE ACTIVE lets the trait be active on your Sideline, it does not limit the use of the trait to your Sideline.
Sidekicks just receive benefits from the above characters. At the moment, they don't grant anything to anyone else (but the Sidekick label may be referenced in traits or special powers).