The stepbrother of Becky Watts started "jumping out at her" to make her scream in the months before her death, a jury heard.
Becky, 16, was allegedly murdered in her bedroom in Crown Hill, Bristol, by her stepbrother Nathan Matthews, 28, and his girlfriend Shauna Hoare, 21.
The pair are accused of targeting 5ft 1in Becky in a sexually motivated kidnap plot on February 19 this year, culminating in her death.
Her dismembered body was discovered in a garden shed in Barton Court - 80 metres from their property - by police on March 3.
Former TA soldier turned takeaway delivery driver Matthews admits killing Becky, dismembering her in his bath and moving her remains to the shed.
Pictures of Becky's bedroom, where she was killed, and the bathroom in Matthews and Hoare's home were released as jurors visited key locations in the trial.
The couple's home is filled with piles of junk but their bathtub - where Becky's body was cut up with an electric saw - was spotless.
Becky's pink-themed bedroom featured make-up, perfumes, clothes, shoes and a T-shirt scrawled with good luck messages from school friends.
In a police interview, Hoare, who denies murder, stated that Matthews had become closer to Becky in the months before her disappearance.
"Recently, Nathan has had this thing, if he can hear her coming down the stairs he will jump out at her," she told police days before her arrest.
"He will jump out at her and she will scream and he finds it hilarious."
Hoare said Matthews was "annoyed" at the way Becky spoke to his mother Anjie Galsworthy - her stepmother.
"Becky gets annoyed at Nathan because she thinks he gets special treatment," she said.
"Becky gets treated amazing. She has a 50 inch TV in her bedroom. She always looks really nice, has lots of shoes and bags and everything.
"In the past month they got quite a bit better with each other.
"If she had any questions about her phone and laptop she would ask Nathan and he would go up to her bedroom and help her.
"They have bonded over the past three or four months."
Hoare described finding Matthews in Becky's bedroom, offering to fix the teenager's laptop and tablet then buy them from her, a few weeks earlier.
"I went upstairs and said 'what are you lot doing up here?'," she said.
"He was there with the laptop. It was a complete surprise. I think he has been up there twice or three times."
She claimed Becky used her anorexia, which she developed after being bullied about her weight, to get her own way.
"I know a lot of people thought she may have been doing it for attention because she got a lot more special treatment like being taken to her new school in a taxi," she said.
"It seemed more of an attention thing than a disorder. She became more confident because she lost weight and started getting more attention from boys.
"She started wearing more make-up and made her feel more womanly."
Hoare said Becky and her father, Darren Galsworthy, would "argue all the time" over her behaviour.
"Her dad used to shout at her and she used to shout at him," she said.
"Anjie told me Darren was taking Becky out of the will because of her behaviour and the way she had been acting."
Hoare added that Becky feared she would be raped, which she claimed was an example of the way she manipulated people.
"She knew how to work people, that kind of thing," she added.
Matthews, of Hazelbury Drive, Warmley, South Gloucestershire, denies murder and conspiracy to kidnap.
He admits killing Becky, perverting the course of justice, preventing burial of a corpse and possessing a prohibited weapon.
Hoare, of Cotton Mill Lane, Bristol, denies murder, conspiracy to kidnap, perverting the course of justice, preventing burial of a corpse and possessing a prohibited weapon.
Donovan Demetrius, 29, of Marsh Lane, Redfield, Bristol, and James Ireland, 23, of Richmond Villas, Avonmouth, each deny a charge of assisting an offender.
Karl Demetrius, 29, and his partner Jaydene Parsons, 23, both of Barton Court, Bristol, have pleaded guilty to assisting an offender after the teenager's body was discovered in their shed.
Both maintain they were unaware of what the packages actually contained.