When Kris Hallenga, the founder of the CoppaFeel! breast cancer awareness charity, was diagnosed with stage four breast cancer, she was 23. Things had initially moved very slowly, as doctors were sure that her symptoms were harmless consequences of the pill and hormonal changes; then, once cancer was suspected, very fast, as they raced to discover its extent.

Almost immediately, though, they found out that there was secondary cancer in her spine, and everything ground to a halt. The questions changed, she told me in an interview three years ago. They became: “‘What is the most important thing as you see it? Pain. Right, let’s fix the pain.’ It’s palliative care at that point.”

Hallenga, who has died aged 38, was mystified by her terminal diagnosis. It became her mission to make other young women aware that such a thing was possible, to familiarise them with symptoms and to spread an everyday vigilance, so that everyone would know what was “normal for them”.

This she did, often with her twin sister, Maren, always with friends, later with other women who had been diagnosed at a young age, in festivals, schools, colleges, clubs and universities, on social and traditional media. Once the sisters’ charity, CoppaFeel!, had found its stride, 1 million young people a year would receive a text message to remind them to check their breasts.

She was born in Lower Saxony, to a German father, Reiner Hallenga, who died when she was 20, and an English mother, Jane, both teachers. At nine years old, following their parents’ divorce, Kris, Maren and their older sister, Maike, moved to Daventry, Northamptonshire, with their mother. Kris left the local grammar school dreaming of becoming an air hostess, and took a job with a travel company, which saw her relocate to Beijing, teaching English on the side.

In 2009, she returned to the Midlands to gain some teaching qualifications. While her GP was not worried about her symptoms, her mother was: Hallenga’s grandmother had had breast cancer before the age of 30, in the 1950s.

Once she received her bleak prognosis, Hallenga’s immediate response was a fierce energy, she remembered later: she was in the middle of her first, very aggressive round of chemotherapy when she started CoppaFeel! at a festival, “in the middle of a field, with no hair, but I still had eyebrows”, handing out stickers and starting conversations.

She won a Pride of Britain award that year, as a result of which she went to a reception in Downing Street and met Sarah Brown, the then prime minister Gordon Brown’s wife, whom she always credited with expediting the process of getting the Charity Commission to register CoppaFeel! In 2018, it was the third most recognised cancer charity among young adults in the UK.

There was an element of displacement activity at the start, she said. “When someone is diagnosed with breast cancer, it’s almost as if a silence, a gap or a moment has to be filled because it’s so heavy and so tragic. The cancer patient usually feels as if they have to fill it, and they have to say, ‘It’s OK,’ when it’s not. I gave my friends something to do. I needed them to help with the charity, they all said ‘Brilliant’.”

The charity gained momentum: text message reminders, mass media campaigns such as the partnership with the Sun newspaper in 2014, school visits from the Boobettes, a growing network of young women with cancer. In 2018 the charity made significant progress in its campaign for cancer education in schools with the government’s announcement that health education would be a compulsory part of the curriculum, with pupils taught “the benefits of regular self-examination”.

The year before, Hallenga was behind the first ever daytime TV ad to show a female nipple, and was particularly proud of getting images of nipples on to billboards for the first time.

She wrote columns for the Northampton Echo and the Sun, and had an open, personal style that was powerfully persuasive. Throughout, she was undergoing periods of treatment that were sometimes utterly debilitating; the cancer in her spine necessitated particularly gruelling operations, and symptomless tumours in her liver and brain often showed up in routine scans. Her oncologist used to call them “dead pigeons”.

She stepped down as the chief executive of CoppaFeel! in 2017, in order to move to Cornwall to be near Maren. It was from Newquay that she wrote her memoir, Glittering a Turd, which was published in 2021 and became a bestseller. She also participated in twin studies at Kings College London and liaised with Tim Spector on the dearth of diet advice for cancer patients. Maren’s son, Herbie, was born in 2019, and Kris relished spending time with him.

While she had stood aside from her charity, she remained deeply motivated by its work, and by her determination to share her message as widely as possible. It was to this that she partly ascribed her longevity following her diagnosis: even though she purposefully did not seek out survival statistics, she could often tell that oncologists were surprised by how well she remained.

She said that each new project and idea “boosted every cell in my body. Every single cell is thinking: ‘We have to stick around for this.’”

She is survived by her mother, and by Maren and Maike.

• Kristin Hallenga, campaigner and writer, born 11 November 1985; died 6 May 2024