Jenny Johnson/G-Girl is a confused, insecure young woman who also happens to be superheroine, thanks to her contact with a mysterious meteorite in my super ex-girlfriend. She was played by Uma Thurman
Her story
After foiling a purse snatcher who tries to steal Jenny Johnson's purse on the subway, Matt (Luke Wilson) becomes Jenny's "hero" and starts dating this shy stranger.
After several dates, Jenny displays increasingly neurotic and aggressive behavior, becoming more demanding and ultimately injuring Matt and destroying his bed the first time they have sex.
Soon after, Jenny reveals to him that she is in fact the voluptuous blonde superheroine, G-Girl, who accidentally received powers such as flight, superhuman strength, speed, and senses, invulnerability, super breath, and heat vision after she was exposed to radiation from a crashed meteorite as a teenager like how she met her childhood friend who later become her nemesis Barry, love Barry and later becoming a super hero even since.
At high school, she got the powers and change her personality, stop seeing Barry and they both got their own way.
Jenny starts to become more controlling after she reveals her powers and Matt starts to lose his mind.
As Matt and Hannah's friendship develops further, and after becoming aggravated with Jenny's escalating jealousy, Matt ends the relationship.
An enraged Jenny vows to make Matt regret the decision, using her superpowers to publicly embarrass him, throwing his car into space and eventually causing him to lose his job as an architect when she strips him naked during an important meeting.
Professor Bedlam (Eddie Izzard), Jenny's former childhood friend and now G-Girl's nemesis, contacts Matt in order to enlist his aid in defeating her.
Matt refuses and makes plans to leave the city. As he does so he is contacted by Hannah who has broken with her cheating boyfriend, and after confessing their feelings to one another, they end up in bed.
Jenny (as G-Girl) discovers them in bed the next day. Enraged and jealous, she attacks the pair with a great white shark. Angered, Matt contacts Professor Bedlam and agrees to help him defeat her, as long as Bedlam retires from being a supervillain.
He instructs Matt to lure Jenny to a meeting where she can be exposed to another meteorite that will draw away her powers, leaving her a mere mortal. Matt agrees and meets Jenny for a candlelit dinner at his apartment, under the pretense of wanting to resume their relationship. Hannah arrives to see Jenny sitting on Matt's lap.
The two women fight, and in the struggle Jenny's superhero identity is revealed to Hannah. Bedlam's trap is sprung, and the energy that gave Jenny her powers is drained back into the meteorite, incapacitating Jenny.
Professor Bedlam appears, but reveals that he has no intention of keeping his promise to retire from villainy and plans to take the powers for himself. While he and Matt fight, Jenny crawls to the charged meteorite attempting to regain her powers.
Hannah intervenes just as Jenny grabs the meteorite, which explodes in a burst of power. Both Hannah and Jenny are catapulted off the roof, apparently to their deaths; Jenny appears within seconds, powers restored, threatening even more mayhem.
Only the unexpected reappearance of Hannah, who was also exposed to the meteorite's energies, and now possesses the same powers as G-Girl, saves Matt. The second fight between Hannah and Jenny is a full-on super-brawl, destroying part of the neighboring properties.
Finally, Matt reasons with them both and they cease fighting. He tells Jenny that Professor Bedlam is her true love. Jenny was surprised that Barry still love her after those years and she return her feelings for her former nemesis.
The next morning, Matt and Hannah meet up with Professor Bedlam (now just "Barry") and Jenny. As cries for help are heard from afar, Jenny and Hannah, who have become partners in crime-fighting, take off to tackle the emergency.
Matt and Barry are left holding their girlfriends' purses and clothes, and leave to have a beer together. At the end credits, she and Barry were married.