I was recently asked to sum up Jo Cox in just one word and what instantly came to mind is a word that doesn’t properly exist. Unstoppability. It seems to perfectly describe the utterly irrepressible nature of Jo as a mighty force for good. Once she had engaged first gear, she accelerated to fifth and stayed there – always in a straight line but with room for manoeuvre and usually making lots of noise – until that particular journey was over. Reverse was never an option and the next journey was always in planning. Jo was relentless and got things done, almost always with a genuine smile on her face. But behind that smile lay a purpose.

Climber of hills and mountains. Every challenge higher than the last one. That is the Jo Cox I will remember

I had already heard about her before our first meeting. At Oxfam, where she worked until 2009, she was rightly considered a treasure – that rare high-achiever who everyone from bottom to top could relate to and I knew that she had also worked for Glenys Kinnock, who was a huge fan. Perhaps that natural ability to connect had been honed at Cambridge University, where her confidence took knock after knock, but she was never defeated and emerged so much stronger. By the time we met, she was making waves. I was told: “You will like her.” And I did.

Jo was wildly enthusiastic and cheerful and so persuasive in getting people to join our cause. I think it says so much about Jo that she changed her name three months before she married Brendan as she started her new job working with me, because there would be no fiddly paperwork distractions that might interfere in her new marriage and her new job.

Jo Cox, photographed by the Thames, where she lived on a narrowboat. Photograph: Family photo

While I lived and worked at 10 Downing Street, Jo became the director of the maternal mortality campaign I had founded with the White Ribbon Alliance and we worked closely together. She had great experience as an aid worker and had spent time in Bosnia getting to grips with the changes for women caught up in conflict. Few campaigns bring the reward of success and ours had been long-fought by many women but Jo and I had joined forces at a time when the door was ready to open. We worked during a time of great breakthroughs in reducing maternal deaths and both of us emerged with a genuine belief that change was always possible. Jo’s role was to charm people into submission and find common ground, even where others thought their differences were insurmountable. I like to think I’m good at doing that. Jo was better.

The memories that I will never shake are of Jo, pregnant in New York with her son Cuillin, running around lining up a panel of VIPs to speak up for pregnant mothers, Jo back in London with baby Lejla perched on her hip at a Labour Women’s Network dinner while giving a brilliant speech in praise of feminism and activism, and finally a vivid image of Jo, with Brendan, sitting in our home in Scotland after the 2010 election – sharing, talking, planning, positive – not a hint of looking back.

It goes without saying that her spirit will continue to shine brightly through her loving family, her close circle of friends, political colleagues throughout the world and the protective community of Batley and Spen. Her loss has been crushing for so many.

One thought has struck me. For someone who lived on a boat on the flat calm of the Thames, she was incredibly passionate about the British countryside and, in particular, climbing. That is the Jo Cox I will remember. Climber of hills and mountains. Every challenge higher than the last one. And even when she got to the top there was the next one and then the one after that. Jo and her unstoppability.