Witnesses testified Breivik seemed fairly normal until 2006, when he moved in with his mother and became reclusive, as well as addicted to online gaming. They said they thought he might be depressed or was perhaps gay, The Daily Telegraph reported.
They also said they were pleased when he moved to a farm in rural Norway, not realizing he was building a bomb.
Breivik, who has admitted to killing 77 people last year in a bomb-and-shooting massacre, responded to the testimony. He said he pretended he was addicted to World of Warcraft and denied he had tortured a cat or was vain because he wore makeup.
"That is also wrong. I have not been vain after 2006 and I've never been feminine either," he said.
He also denied his friends' testimony that he seemed depressed. "I can confirm that I have never been depressed in my whole life," he said.
Breivik's family members won't be called to testify at his trial.
Breivik's mother has been excused for health reasons and his step-mother and sister can't be forced to take the witness stand to testify against a relative, Norwegian Broadcasting reported Tuesday.
Breivik's lawyers said family members aren't on the witness list for the defense at Breivik's own request.
Only four of Breivik's former friends were scheduled to testify.
"We have summoned these friends because they can help describe how Breivik was as a person before July 22," said prosecutor Svein Holden.
All four have known Breivik since their school days.
Breivik's sister has already told police she doesn't think her brother is insane.
Transcripts indicate Breivik's sister told police her brother "understood what he was planning, and knew the difference between right and wrong ... you can't be sick and plan what he did," Norway's Aftenposten reported.
Breivik confessed to the July 22, 2011, killings in a bomb attack at a government building in downtown Oslo and a shooting spree on nearby Utoya Island but pleaded not guilty to murder and terrorism charges. He has claimed his actions were justified because he was waging war against multi-culturalism and what he termed a Muslim invasion of Europe.