Halfling Rogue, Sniper, Burglar, Investigator, or Spy Archetypes
To feel important, all she needs is utility. In and out of combat. If she's hiding and sniping or tripping and sneak-attacking, or even just flanking and moving with Spring Attack to provide bonuses and then vanishing, it's all useful combat tactics without being a frontline fighter.
If she's the one that sorts out the treasure, identifies it, and finds the hidden traps/secret doors, she'll be very VERY important.
In a whodunit campaign, rogues are useful for Bluff/Sense Motive. She could be the interrogator of captives.
Ninja, maybe?
City. Corrupt city, corrupt enough a childlike halfling might witness a murder, and be pursued, a villain shouting, 'seize that child!' the first time she escapes, 'kill that child!', the second time. :) And 'KILL! THAT! HALFLING!' the third time.
A city un-corrupt enough that a honest cop/soldier/sailor/barmaid can be found, and know where to take her to make witness.
A city big enough that it's far enough travel for a two-hour movie. :)
If she tries just to hide, they find her, or they come so stupidly CLOSE to finding her, that you have a scene like the leader pacing in a room past the table under which she's hiding, while he rants of his plans...
The cop/barmaid has to be tough-ish. Tough enough to help, and to rescue the halfling. Weak enough to get overwhelmed themselves, and need rescuing in turn.
Because you said important. And all that, that's important. Not the kind of important the villain thinks HE is. But the 'hero of the narrative' kind of important.
If you're going to suggest bard instead of rogue, I believe along with the Archaeologist, there's also a Detective archetype. Might fit your whodunit campaign.