Psychologist Anna Salter testified in what is expected to be a two-day sentencing hearing, KUSA-TV, Denver, reported.
Salter called Sigg "sadistic" and "narcissistic" when he kidnapped Jessica Ridgeway, 10, noting his behavior was "extremely rare" for a teenager in that "considerable care was given to avoid detection."
The sentencing hearing began Monday, and is expected to take two days.
Ridgeway was last seen walking to her Westminster school in October 2012. Her partial remains were discovered five days later in Arvada, after Sigg's mother called police, saying her son admitted to the crime, KDVR-TV, Denver, said.
Last month, Sigg, now 18, defied the advice of his attorneys and pleaded guilty to all 14 charges against him, including murder, in the death of Ridgeway and for the attempted kidnapping of a jogger in May 2012, KUSA-TV, Denver, said.
It is likely Ridgeway's family will address the judge before the sentencing phase of the trial ends, KUSA-TV said.