Every Thursday just after midday Hajar Rostami leaves her small flat in the east of Tehran and makes the 12-mile journey to Behesht-e-Zahra cemetery on the outskirts of the Iranian capital.

When she arrives the 48-year-old picks her way through the sea of tombs and headstones until she reaches the grave of her middle daughter, Neda Agha-Soltan.

There are normally people gathering around the simple monument and Rostami passes round a notebook so that they can write down their memories of her daughter, before laying fresh flowers and tending the grave.

But even here the mourners are not left in peace. At a discreet distance, two officials from the Iranian secret police watch as the scene unfolds.

It is almost a year since Rostami's daughter was killed during an anti-government protest in her home city of Tehran. Her death has transformed the lives of the family she left behind.

Neda's final moments were captured on jerky mobile phone footage and within hours the grisly images became the defining moment of the thwarted uprising against the Iranian regime – and a rallying point for a movement in need of a hero. Since then Neda – and her very public death – have become an internet phenomenon and her image has been printed on placards and brandished at demonstrations around the world.

But back at the small first-floor flat that she shared with her mother, father and brother it is clear that Neda's posthumous fame has not eased the family's pain. "We were not mother and daughter," says Rostami as she busies herself in the kitchen. "We were two friends. Anything that happened, she used to come to me."

She pauses before adding: "I used to tell her that if I'm ever ill, bring me to Shariati hospital. I never believed that we would be bring her to Shariati hospital to die. I used to tell her to bring tuberose flowers to my grave. I never believed that I would be bringing tuberose flowers to her grave."

During the month that Guardian journalist Saeed Kamali Dehghan spent with Neda's family to film the documentary For Neda, a picture emerged of a typical young Iranian woman. Neda enjoyed new fashions and make-up, she went to the gym and loved dancing. She was fiercely independent and a voracious reader whose favourite novel was Emily Brontë's Wuthering Heights.

Sitting around the kitchen table, this is the way her family remember her – not as a symbol of the resistance but as a dynamic and determined woman, quick to laugh and quick to stand up for what she believed in.

"She was always very brave and defended herself," says Rostami. "She was given the nickname Tayeb [after a 1970s South Tehran street tough]. All the children in the streets were afraid of Neda, because she was so strong. Even her older sister, Hoda, deferred to her."

Neda married her first love, Amir, in her early 20s but his family's social conservatism grated against Neda's more liberal background, and the couple drifted apart.

They remained on good terms, but eventually divorced, and Neda returned to the family home, where she grew close to her 23-year-old brother, Mohammed. They shared a passion for music – he used to play the guitar and she used to sing. A few weeks before she died they went shopping for a piano that he now plays twice a day "to remember her" – once in the morning before leaving for work and once at night.

"I can't still believe that she's gone – when I play the piano I feel she's here in this room," says Mohammed. "First we were brothers and sisters but in recent years we were like two friends, we had the same group of friends, we used to go to same parties."

Until last year's disputed elections Neda had never shown any interest in politics and she and Mohammed used to spend time dancing and going to underground parties. Since his sister's death Mohammed has vowed not to cut his hair or beard.

"On the last day of her life, in the morning, we met each other and I had no time to shave and at that time my hair was longer than usual. She said that she liked me this way, with a beard and long hair. Since her death, I couldn't shave neither my hair nor my beard."

Neda's death has also left a hole in her mother's life. Unusually in Iranian culture she has decided to keep her daughter's bedroom exactly as Neda had left it: Neda's favourite teddy bears are scattered on the bed, a poster of Dire Straights guitarist Mark Knopfler hangs on the wall, her make-up bag is open on the dresser.

But the political circumstances surrounding her daughter's death have also had an impact on Rostami. As an Iranian housewife concerned predominantly with her children and their wellbeing she had never shown much interest in politics. Over the last 12 months she has been thrust into the maelstrom of opposition politics in Iran.

"I'm not a politician, I'm a very ordinary Iranian mother, but since Neda's death I had to keep myself updated by the political news, I can't remain indifferent to what's happening in my country."

Rostami now speaks to other mothers who have lost children at the hands of the authorities, attends women's rights meetings and gives online interviews.

"Because of the world attention to Neda's death, her story is spread around the world," she says. "But what about the story of those others who were killed, raped and tortured after the election and nobody knows about?"

The authorities take a dim view of her new life and Rostami suspects her home is watched and that the regime monitors her phone calls and keeps tabs on who she meets. But the family believes the prominence of the case means the government has no choice but to keep its distance.

"The more the government tries to stop people from remembering Neda, the more her story spreads among people," says Rostami.

Neda lies buried at Behesht-e-Zahra, Iran's largest cemetery, and a place synonymous with the country's "martyrs" – the hundreds of thousands of young men killed in the Iran-Iraq war.

Her grave has been desecrated several times. But Rostami remains defiant. She says she will not be intimidated and claims the regime is scared her daughter is becoming a martyr for a new generation. "Although Neda has been murdered and is dead, they are still afraid of her, they come to the graveyard and want to kill her again. She's dead but her memory is getting brighter and brighter every day."