Carer Stephen Nicholson, of no fixed address, is accused of repeatedly stabbing the 13-year-old to the neck and upper body at Southampton Sports Centre in July last year.
Pathologist Dr Basil Purdue told Winchester Crown Court that Lucy's death was caused by 11 stab wounds to the right of her neck which cut a main artery three times.
The court was told that Lucy suffered a total of 20 stab wounds and seven sliced wounds to her face, upper chest and forearms caused by a single-bladed knife at least 7cm long.
Dr Purdue said: ‘The cause of her death was catastrophic bleeding from a series of 11 injuries to the right side of the neck that punctured her right carotid artery in three places.
‘This would have been rapidly followed by further sharp injuries to the face, neck, chest and lower forearms.’
He added that one injury, a 3cm sliced injury to Lucy's left wrist area, appeared as if it could have been inflicted deliberately in a bid ‘to divert suspicions away from homicide’.
The trial has heard that the defendant told police that Lucy had sent him a message saying she was pregnant but Dr Purdue said that blood tests showed that she was not pregnant.
Richard Elmes, the partner of Lucy's mother, Stacey White, said that Nicholson had been an occasional lodger at their home in Mansel Road East.
He said that he had not thought that Lucy was sexually active and when he and her mother confronted her about comments at school about her being pregnant, she had told them that she had told her friends that her mother was pregnant.
Mr Elmes said that he had considered Lucy and the defendant as being like part of a ‘normal family’ with ‘petty’ arguments.
He said that he believed Lucy had a ‘schoolgirl crush’ on Nicholson and in May 2018, he walked in on ‘Lucy giving him a hug’ before the defendant pushed her away.
Mr Elmes said that a week before her death, Nicholson and Lucy had an argument and another like a ‘war zone’ on July 23 during which she shouted at the defendant ‘I have got a hold on you anyway’.
He said: ‘I told him that basically he had to grab his stuff up and he had to leave because the situation wasn't getting any better.’
Describing Lucy, he said: ‘She was always a lovely, bubbly, bright, intelligent girl, she knew what she wanted and how to get there.
‘She was very particular about how she looked. She would always like to be the centre of attention, always liked to know what was going on and how people are.’
Mr Elmes said he had known Nicholson since he was six years old but their friendship had halted while the defendant had spent a period in prison.
He added that the defendant kept six snakes and a tortoise at their house and started his hobby of tattooing after he had come out of prison ‘to keep him away from the old life’.
The 25-year-old defendant is charged with Lucy's murder as well as three counts of rape against her when she was aged 12.
He also faces two charges of sexual activity with a child against Lucy on multiple occasions when she was aged 13.
Nicholson is also charged with sexual activity with a child in relation to another girl aged 14, dated June 29 2012.
The defendant denies the charges and the trial continues.