In the ladder of roleplaying game assistant characters, the sidekick is one rung above a henchman, who is a rung above a retainer. The sidekick can be expected to have a complete character sheet, and even possibly a personality.

Potentially more personality than the PC they're sidekick to.

Transcript

GM: Obi-Wan, you chat to a guy called BoShek.
R2-D2: Is this your new character, Jim?
Captain Antilles: {dead} No, not yet.
BoShek: Pilot, huh? This Wookiee should be able to help you.
R2-D2: Is that your new character?
Captain Antilles: {dead} Nope! He's my sidekick. How cool is that? Is my character background detailed or what?
Captain Antilles: {dead} Not only that, it's Chewbacca!
C-3PO: Cool!
{beat}
R2-D2: Who?
Captain Antilles: {dead} You know, the Wookiee that Yoda talked to back on Kashyyyk!
{beat}
C-3PO: That's even the correct pronunciation!
Obi-Wan: Who are you and what have you done with the real Jim?