Hey, Thor has loads of sidekicks—why shouldn't his girlfriend have one too?
Darcy is Jane's intern, there to take notes, carry heavy equipment and do all of the other things academic types require. As such she's basically along for the ride, there to provide a few moments of comic relief like when she first spots Thor's overall hunkiness.
DARCY: Does he need CPR? Because I totally know CPR!
It's a tough job, but somebody's got to do it. And with Jane still not sure whether Thor is barking mad or not – and with Erik far more mentor than friend – she needs a good buddy like Darcy to back her play… just like Thor does.