The male teenager, aged under 18 at the time of the stabbing, has pleaded not guilty to the murder of Balin Stewart, aged 16, just metres from his family home in the Sunshine Coast suburb of Buddina on the night of January 20 last year.
The Brisbane Supreme Court heard on Monday that Balin died from a single stab wound to the heart from a steak knife with a 12-centimetre blade.
The jury on Tuesday heard testimony from the defendant's ex-girlfriend, who was also under 18 at the time and cannot be named for legal reasons, after viewing the girl's two conversations with police in the hours and days after Balin's death.
The witness was shown multiple silent videos she made of herself speaking directly in front of her phone's camera, which were sent to the defendant before the stabbing, but she told crown prosecutor Rebecca Marks that she could not recall what she had said.
The jury was also shown other videos sent to the defendant, including a video in which a shirtless Balin embraced the witness from behind, a video of the witness touching Balin's exposed buttocks and video of the pair lying on a bed while clothed.
The defendant sent messages in response "Where R U?" and "Balin you wanna fight?"
Before driving to Balin's house and confronting him in the street, the defendant also texted the witness asking, "Have you sucked any more guys off?"
Under cross-examination from the defendant's barrister, Charlotte Smith, the witness agreed that word had spread through her social circle about her relationship break-up with the defendant in the weeks before Balin's death.
"You were aware that everyone was calling (the defendant) dumb for cheating on you? Is that accurate?" Ms Smith asked.
"Yes," the witness replied.
She said she could recall being told six days before the stabbing about an image circulating in a social media group chat of Balin and another boy holding knives.
The witness said she consumed two cans of Smirnoff Ice Double Black pre-mixed vodka cans at a party and a third can in Balin's room before he was stabbed.
"I was happy but not drunk," the witness said.
The witness said he could not recall whether she was present when a friend used her phone to call the defendant and warn him to be careful after cheating on her.
She agreed she felt guilty about making the defendant upset by sending him videos of herself with Balin but denied she had lied to police to conceal the content of the videos.
"There were a lot of things going through my head. There was a lot I missed out on," the witness said.
She is scheduled to continue giving evidence on Wednesday.
