Arsema Dawit, 15, whose body was discovered in a lift in a block of flats near London's Waterloo station, had been attacked before, Simon Tesfaghiorgish told reporters.
Mr Tesfaghiorgish said: "I didn't know him but he had been harassing her. The mother told the police a number of times. The police said they couldn't take any action."
He added: "We are going to complain about this. He beat her and threatened to kill her."
Police sources confirmed officers investigated a complaint of assault on Arsema in April. It is understood that no one was arrested.
Arsema's body - still clothed in her school uniform - was found slumped in a lift in Matheson Lang Gardens at around 4.30pm on Monday. She had been stabbed repeatedly.
A man in his 30s, who was arrested on Monday night, was being questioned about the murder at a south London police station.
Neighbour Wayne Fort, whose partner and nine-year-old daughter found the body, said Arsema had been troubled in the past by a man.
Mr Fort said his daughter was extremely upset by the discovery she made. "She is in a very bad state - she didn't sleep much last night."
The murdered teenager had celebrated her 15th birthday with her family on Saturday, he added.