Aliza Mirza, 18, kept the relationship from her strict Muslim parents because she feared they would disapprove, friends said.
Last night police were still questioning 17-year-old Yahya Gul, believed to be the ex-boyfriend, on suspicion of murder. Several members of his family are also being held.
Aliza was knifed in the neck and left to die on a pavement on Saturday evening.
On Facebook Gul called himself ‘Geeneral Yahya’ and listed his religious views as ‘Muslim till I die and here after’.
Friends said he had become extremely religious since dropping out of school because of non-attendance. He had started wearing traditional robes and grew a beard.
Ali Razza, 18, said: ‘I was friends with both of them, I got to know him through her.
‘They met through Facebook and carried on from there. Her family were very strict, so the relationship was a secret.’
Aliza’s distraught family yesterday paid tribute to the ‘diamond daughter’ who would do anything for them, but insisted she ‘never had a boyfriend’.
Breaking down at the family home in Ilford, East London, her father Munir Mirza said: ‘She was pretty, intelligent, she had very bright future. She was very interested in the media. She wanted to be a journalist.
‘She was a brilliant girl and she had all the qualities that a daughter should have.’
The last time Mr Mirza saw his daughter was when she asked to borrow some money to buy her friend a present ahead of the party.
‘She left here to go to the party at 5pm,’ he said. ‘At 8.30pm the police came here and said she was in hospital and we should go there.
‘Then ten minutes later they phoned and said she was dead. We were too late.’
He added: ‘We never heard anything about her having a boyfriend. We didn’t know anything like that, nothing at all. She never had a boyfriend.
‘She was a diamond to me and my family. She was everything to me, to her uncles, aunts, the whole family. Everyone loved her so much.
‘We are not going to touch her room, her laptop, nothing. It will all stay as it is.
‘We want to do that for her. Whoever did this should get punishment from the courts.
Aliza’s uncle, Waqar Ahmed, 43, said: ‘I just can’t imagine why anyone would want to kill her.
‘She’s not someone who could harm anyone, she was very petite, very small. She was really fun-loving. ‘
Aliza, who had two brothers and two sisters, was studying ICT, English, government and politics and media at Canon Palmer School in Ilford.
Yahya Gul’s 45-year-old father Ilham is also being questioned on suspicion of murder.
Gul’s mother Suraya and his 20-year-old sister Sara are being held on suspicion of assisting an offender, along with another man aged 42.
Neighbours said Sara Gul was due to be married today and that relatives started arriving at the family home yesterday morning oblivious to the arrests.
Detectives have recovered a weapon which they believe was used to carry out the stabbing. They spent yesterday searching the Victorian house rented by the Guls.