After Odysseus eu Britannia had returned back to the Britannian homeland after the coup in the Chinese Federation instigated by Li Xinge of the Chinese Military, the Black Knights had already risen and managed to overthrow the corrup government lead by the High Eunuchs. With the High Eunuchs wiped out, the country began to fall apart and Charles zi Britannia ordered that the fallen empire be annexed immediately. Knight of One Bismark Waldstein delivered the message to Guinevere and other royal family members at a meeting in the Imperial capitol of Pendragon. Guinevere appeared at a meeting in Pendragon where she agreed to the idea of invading and annexing the Chinese Federation along with Carine ne Britannia while Odysseus disagreed citing his lack of hostility towards the Chinese and the ongoing war against the Europia United.
While there was a near-universal consesus on the issue, the problem remained that there wasn't an available commander to lead the Britannian troops for the campaign when Schneizel el Britannia suggested putting two divisions on the Mongolian border and said how the Chinese Federation had already dissolved so military action wasn't needed and would use negotiations to acquire half of their former territory. Sometime later, Lelouch would arrive on Kamejima Island where Charles would go missing and she would be one of the few people to know of his dispearance.
Eventually, Guinevere would appear in Pendragon again this time, awaiting a public announcement from Charles zi Britannia who wasn't present for a long time and was secretly dead. Lelouch appeared instead of Charles during a live international broadcast to the entire nation of Britannia where he claimed himself to be the 99th emperor and that he had survived the Britannian invasion of Japan. Guinevere was shocked along with the rest of the imperial family at Lelouch's presence and were horrified when he declared that Charles zi Britannia is dead and that he killed him himself. Outraged, Guinevere ordered the Royal Guard execute Lelouch when they were stopped by Suzaku and eventually, she and the entire royal family was brainwashed by Lelouch's geass and accepted him as the new emperor.
After Lelouch ascended to the throne, he had the aristocracy officially abolished and had all of the royal family members demote themselves to civilian level ranks with Guinevere and Carine being maids and Odysseus enlisitng in the Britannian military to cite examples. She remained in Pendragon and was last spotted there prior to her death at the hands of Prince Schneizel when he destroyed the Britannian capitol using F.L.E.I.J.A. warheads via the Damocles.