Spoiler warning is in effect as I'm covering the events of Starfall Street.

When we first meet Penny, she's being harassed by Team Star grunts, a quick message from Cassiopeia recruits the player character to overthrow Team Star, throughout the story Penny rewards players for each take down of the leaders of Starfall Street, eventually you'll put two and two together and realise that Penny is the actual leader, her story is a tragic reality to anyone who has ever dealt with bullying, as an attempt to stand up for themselves as a team it cost them their leader as the school botched the whole incident.
Penny has a mean streak built on years of being cynical and it rubs off on the team, she even stole funds from the Pokemon League to pay you the player character.
But she's also an otaku shut in with an awesome Eevee backpack even quoting the fact that she binge watches anime regularly.
She also has the best battle music.
To end with, a wild theory, Penny looks like she could easily be mistaken for the daughter of Jesse and James