However, when Bill's love interest Heather (Stephanie Hyam) is turned into a water creature that can travel anywhere in space and time, the Doctor reveals to Bill his alien nature and takes her on as his travelling companion, much to the chagrin of his assistant Nardole (Matt Lucas), who urges the Doctor to stay in Bristol and keep watch over a strange safe below the university.

When the Cybermen are defeated, Bill is entirely alone: the Doctor appears dead, Nardole has taken Mondasian refugees to safety, and she is a horrific monster.

She is rescued, however, by Heather, who transforms her into a creature like herself and offers Bill the choice of travelling space and time together, or being returned to her life on Earth as a normal, living human again.

Mackie makes a final appearance in "Twice Upon a Time" as a living avatar of Bill's memories, collected on her death by the Testimony Project.

In the 60th anniversary special story "The Giggle", The Toymaker depicts Bill as a marionette alongside Amy Pond and Clara Oswald, taunting the Fourteenth Doctor by reminding him of companions who died while travelling with him.

[4] Before her premiere episode was broadcast, Bill made a cameo appearance in a Doctor Who Magazine comic strip, titled "The Daft Dimension", alongside Nardole in Issue #511 (May 2017).

As they spend time together on Earth – magicking up enough money to rent a flat and adopting cats – Bill grows to know and love Heather as a person.

Bill challenges the Doctor on his ways, calling him out on issues that he hasn't faced in a long time, and having a curious mind, she asks him continuous questions.

[citation needed] Patrick Mulkern of Radio Times described Pearl Mackie as "instantly winning as fledgling companion Bill".

Wilkins also commented, "that she is gay, black, and working class is another welcome step in Doctor Who's ability to reflect the entire spectrum of [those] who enjoy the show and identify with its characters".