Bonnie was the Zygon leader of Truth or Consequences, a radical group of young Zygons that opposed Operation Double; feeling Zygons should be free to show who they really were instead of being forced to hide as humans.
Taking the form of the Doctor's companion Clara, Bonnie attempted to manipulate him into deathtraps, but failed miserably. Eventually, she ended up in a stand-off with Kate Stewart with the Osgood Boxes. In the end, the Doctor managed to help Bonnie see that she wanted nothing more than a senseless cycle of violence; it was better to protect others and try keeping other lunatics from killing.
Taking this new purpose to heart, Bonnie decided to take the vacant spot left by the Osgood killed by Missy. Both of them promised the Doctor that they would reveal which of the Osgoods that Missy killed, but only when whether if it was the original or the Zygon doesn't matter anymore. From this point onwards, she was known as Petronella Osgood.
Bonnie was raised by a single parent and went through a reproductive phase. (AUDIO: Narcissus) After Zygor's destruction in the Last Great Time War, (TV: The Day of the Doctor) Bonnie lived in a Zygon colony on Earth. (TV: The Zygon Invasion)
Truth or Consequences
While Bonnie (still in Clara's form) led Jac and some UNIT soldiers into the Zygon colony, she dusted off the Zygon pods in which they were keeping the kidnapped humans. Jac quickly deduced that Bonnie was not actually Clara and that it was the humans that were in the pods. Bonnie ordered her fellow Zygons to kill them.
Later, overlooking the sea, Bonnie prepared to fire an FIM-92 Stinger missile launcher at Boat One with the Twelfth Doctor and Osgood inside. The Doctor attempted to reason with her over the phone, but Bonnie, despite the Doctor's protests, fired a missile at the plane. (TV: The Zygon Invasion)
Clara, maintaining a live feed to Bonnie from inside a Zygon pod, caused Bonnie to misfire her bazooka, allowing the Doctor and Osgood to escape.
Bonnie later had some of her group murder humans and sweep up their remains. She chased Etoine to his flat, knowing he was a Zygon. She broke into his home and told him that he would be the first to make the humans see them for who they were. Giving him a zap, Bonnie disrupted Etoine's shape-shifting ability, and recorded him turning back into a Zygon. She posted the video on the Internet.
Bonnie remained unaware that this made Etoine commit suicide.
Later, Bonnie was able to extract the location of the Osgood Box inside the Black Archive from Clara's mind. Both Bonnie and Kate were ready to set off the Osgood Box. The Doctor intervened, the horrific memories of his actions during the Time War and the agony stemming from that not allowing him to let any more suffering happen. After the Doctor convinced Bonnie to not set off the Osgood Box, which was actually empty, Bonnie also assumed Osgood's form and acted as the second protector of Earth alongside the other surviving Osgood. She felt that they were a credit to both humans and Zygons and planned to take on the role of protecting the Earth from threats. (TV: The Zygon Inversion)
Working for UNIT
The Osgoods created a series of files documenting alien species and foes encountered by the Twelfth Doctor beginning with the Zygons and their involvement in Truth or Consequences. Other files documented King Hydroflax, the Mire, The Veil and the Sandmen. (PROSE: The Osgood Files)
In September 2015, while watching news coverage with Kate in the Tower of London, the Osgoods witnessed Daniel Claremont transform into a Sea Devil on live TV, prompting them to suggest calling the Twelfth Doctor for help. However, they soon discovered that he was already dealing with the situation. (COMIC: Clara Oswald and the School of Death)
While one Osgood remained at UNIT HQ, the other asked the Twelfth Doctor to help stop a K2 robot from rampaging across London. She took a brief trip in the TARDIS and saved the Doctor from falling from the TARDIS with her scarf. (COMIC: Robo Rampage)
Together, they decided to investigate the Narcissus dating website and find Josh. Bonnie took the form of Jacqui McGee using her Narcissus profile and met with Jordan Love, who appeared as someone she knew during her reproductive days. However, Jordan soon discovered that she was an imposter and attacked her with a burst of energy that would’ve killed a human, but being a Zygon, she was able to survive albeit injured.
Meanwhile, Petronella prepared to confront Jordan Love on their spaceship. Bonnie tried to stop her, believing that she was more disposable than her sister. Petronella went ahead with the mission anyway and unwittingly defeated Jordan due to her complete self-reliance and contentment being toxic to them. Josh was rescued and returned to UNIT. Following these events, Bonnie and Petronella reconciled, agreeing that two Osgoods are better than one. (AUDIO: Narcissus)
One of the Osgoods was picked up from UNIT HQ by the Twelfth Doctor while the other had gone out to pick up some coffee. The Doctor took this Osgood on her first trip in the TARDIS and asked her to prepare a briefing for him after they arrived on Nayspiel, the capitol mega-city of the Alliance. They stopped a plot by the Lisban corporation to invade the planet Albensi. The Doctor returned Osgood to UNIT HQ just in time to see the return of her sister. (AUDIO: Field Trip)
In October 2016, the Osgoods investigated Saxon Heights boarding school near Devil's End and discovered that Missy had taken over the school as part of a plot to summon a Dæmon. They sent a UNIT team to arrest her, however she escaped soon after their arrival. (PROSE: Yes, Missy)
When Jenny's bowship crashed into the library at St Luke's University, one of the Osgoods was sent to examine the wreckage. However, she was infected with white energy from the Void in the process. Spreading the infection to the other UNIT troops, she was slaved to the will of a rogue Type 1 TARDIS before the Doctor managed to convince the Type 1 to free those under it’s control. (COMIC: The Lost Dimension)
After UNIT's closure
Bonnie was mentally unstable and seemed to suffer from emotional suppression. She often bottled up her feelings, resulting in a calm and cold demeanour but also had an explosive temper. She was smug and sadistic, saying that she was looking forward to hearing Clara scream (TV: The Zygon Inversion) and smiling as she watched her fellow Zygons murder Jac and her soldiers. (TV: The Zygon Invasion) Bonnie seemed to consider murder and war to be a game and was eager to cause bloodshed. (TV: The Zygon Inversion)
However, Bonnie was also childish and when angry, she had a tendency to smash things. Her childishness was also shown when the Doctor pointed out that she didn't actually know what she wanted to do if she took over the world. Having not actually thought that far ahead, Bonnie was unsure how to respond and the Doctor managed to talk her out of starting a war by telling her he forgave her. Bonnie felt remorseful and couldn't understand how the Doctor could forgive her. Feeling she had to try and make up for her crimes, Bonnie became Osgood's "sister" and decided to help her protect the Earth. (TV: The Zygon Inversion)