The jury heard testimonies from members of the university's physics faculty and other witnesses, as defense attorneys continued their case in Christensen's sentencing hearing Thursday.
Witnesses described Christensen as a driven and reliable doctoral student, who was even considered a top teaching assistant.
The witnesses also testified that Christensen eventually became distracted in the months leading up to the 2017 kidnapping and death of Zhang, a Chinese scholar visiting the University of Illinois.
Professors said Christensen missed office hours, skipped out on meetings and even misplaced a student's final exam before dropping out of the Ph.D. program.
Defense attorneys have been attempting to humanize Christensen for the jury, which will decide whether he should be executed.
Christensen's father, Michael Christensen, took the stand Wednesday. He recounted his son's family life and childhood, which included recurring bouts of night terrors and waking nightmares that continued into adulthood.
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The defense argued these "mitigating factors," including addiction and mental illness, should spare Brendt Christensen of the death penalty.
Michael Christensen told the jury he still loves his son, prompting the convicted killer to tear up in court.
The father also apologized to Zhang's family.
"I'm sorry my son was the cause of your pain," he said.
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