Viviane Wembly is a British academic and one of the central human figures in Transformers: The Last Knight. She holds the title of Doctor and works as a professor of English literature at the University of Oxford, where she has also earned a master's degree in history, a doctorate in philosophy, and a doctorate of letters. In addition to her scholarly career, she is an accomplished polo player.

As a child, Viviane was close to her father, who often read her stories about Merlin and King Arthur. The two eventually had a falling out over a three-headed dragon pendant, and by adulthood Viviane had come to regard the Arthurian legends with deep skepticism, privately dismissing tales of King Arthur and the Knights of the Round Table as nonsense. She lives in England, where her mother and aunts are still part of her life, and her father has died before the events of the story begin.

Viviane's ordinary life is upended when she is abducted by the Transformer Hot Rod, acting on the orders of the eccentric historian Sir Edmund Burton. She is brought to Burton's castle, where she meets Cade Yeager. Burton explains that Transformers have secretly been part of human history for centuries and reveals two extraordinary facts: Cade is the legendary last knight, and Viviane is the last living descendant of Merlin. Because of her bloodline, she is the only person who can retrieve and activate Merlin's staff, a powerful Cybertronian artifact. Together with Cade, she becomes central to the effort to stop the approaching planet Cybertron from draining Earth's life force and destroying the world.

At first, Viviane is understandably irate about being kidnapped and is quick with sharp, dismissive comments, trading petty insults with Cade when they first meet. Her opinion of him softens after Burton recounts the personal tragedies Cade has endured. As the mission unfolds, she moves from reluctant participant to decisive actor. Her academic knowledge proves useful when she recognizes Celtic runes on the floor of an ancient Cybertronian ship, and her presence causes Merlin's sarcophagus to rise. When a seemingly wooden staff is found, it is Viviane who grabs it, causing it to shed its holographic disguise and reveal its true Cybertronian nature. Facing down armed opponents, she threatens to drop the staff into the depths of the ship unless they stand down, showing nerve under pressure. When the corrupted Optimus Prime, known as Nemesis Prime, demands the staff and makes clear he would kill her to get it, a terrified Viviane reluctantly hands it over at Cade's urging.

Her personal courage becomes most evident in the final stages of the conflict. When Cybertron begins siphoning Earth's energy, the plan requires Viviane to enter the ignition chamber and remove the staff. After her transport crash-lands and heavy fire pins down the human forces, the military orders an evacuation. Viviane removes her parachute and runs back into the chamber, with Cade following her. Once Quintessa is defeated, Viviane manages to pull the staff from its resting place, stopping the transfer of Earth's energy and saving the planet. She is rescued from the collapsing chamber by Optimus Prime, who has regained control of himself.

Over the course of the story, Viviane develops from a skeptical academic who dismisses Arthurian myth into someone who embraces the reality of her legendary ancestry and accepts the responsibility it carries. Her relationship with Cade Yeager also grows from mutual irritation into reluctant partnership and finally romance, with Cade asking her out during the journey to Cybertron and Viviane agreeing to the date after the crisis is resolved. She also shares meaningful connections with Sir Edmund Burton, who acts as her guide into the hidden history of the Transformers, and with Hot Rod, Bumblebee, and Cogman, the Autobots who protect and assist her along the way.

Viviane has no combat training or superhuman powers. Her notable abilities are intellectual: a deep knowledge of history and legend, the ability to read Celtic markers and clues, and the scholarly instinct to piece together the past. Her most important trait, however, is biological rather than learned. As the last descendant of Merlin, her DNA is the key that unlocks Merlin's staff and tomb, making her uniquely irreplaceable in the effort to save Earth. Her bravery, stubbornness, and growing sense of duty allow her to live up to that inheritance, and her final act of removing the staff cements her role as a decisive figure in the film's resolution. In keeping with the film's heavy use of Arthurian legend, her name is likely intended to echo the Lady of the Lake, a figure closely tied to the Merlin mythos.