From publisher blurb:
They are the loyal followers of mighty heroes, firm believers in the cause, sworn to repay a PC for his deeds with his life while offering no incredible skills. In time, they become an equal force within the party and contribute that unique element to the campaign to make it one of your most memorable ventures.
Sidekicks are a special role in that they are minions designed to aid the PCs. Typically beginning as NPC guides, packmules, and sources of comedy relief, a sidekick can become a precious member of the party over time and can learn to take on the rigors of a class to fight side-by-side with the rest of the party... possibly even becoming a player character themselves.
This special edition of Combat Advantage provides complete rules for creating sidekicks in your campaign, along with three examples and rules for expanding sidekicks into class-based NPCs.