Diane (The Halloween Apocalypse)

Diane worked at the Museum of Liverpool in 2021. She was a friend of Dan Lewis, who often referred to her as Di.

Early life
Though she later refused to call it such, Diane went on a date to a restaurant with a man. She fell asleep in her pizza and woke up with him picking bits of mozzarella off her cheek, while asking the waiter to call an ambulance. She lied and told him she was narcoleptic. Soon after, Diane bumped into Dan as she was leaving work, where he was holding "a skinny latte with an extra shot", just the way she liked it. The pair went on a walk around Liverpool. She told him about her date, and later asked as to why he wasn't married. (TV: Once, Upon Time)

The Flux event
On Halloween 2021, she chastised Dan for impersonating a guide at the museum before arranging to meet him for drinks later that night. Dan agreed, but was abducted by Karvanista before he could attend.

While waiting for Dan as agreed on the corner of Lark Lane, Diane was pulled into an empty building by Azure, who told her: "We're going to have fun with you". (TV: The Halloween Apocalypse) They placed Diane within a Passenger form, in order to use her as a hostage. (TV: Once, Upon Time)

Dan attempted to phone Diane when he returned to Granger Street at 20:10 on 2 November, but he was unable to get through to her. (TV: War of the Sontarans) Later, in the Temple of Atropos, Swarm taunted Dan by revealing her to him. Dan was unwilling to leave the Temple, singularly focused on rescuing Diane, but the Thirteenth Doctor explained that she had left with the Ravagers. (TV: Once, Upon Time)

While trapped inside the Passenger, Diane encountered a fellow hostage named Inston-Vee Vinder. She was pleased to see that Vinder had a gun, and the two of them planned to escape. (TV: Survivors of the Flux)

Diane and Vinder were later rescued from the Passenger by the Doctor and reunited with Dan. By Diane's suggestion, the Doctor later used the Passenger to absorb parts of the Flux that got through the Lupari's shield guarding Earth. Diane later returned to working at the museum where she once again discovered Dan acting as a tour guide. Dan wished to make up for not being able to have a drink with Diane. Due to her experiences however, Diane turned down the offer and returned to work. (TV: The Vanquishers)

After the Flux
Following an adventure in 1807 China, Dan phoned Diane. (TV: Legend of the Sea Devils)

Behind the scenes

 * While her surname was never officially said on-screen, nor in the credits, sources behind the scenes gave her full name as "Diane Curtis" This would also be the name given on her Museum of Liverpool "staff"-card, which is not seen close-up.