For the first time, the only woman to have had an intimate relationship with Colorado theater shooter James Holmes has spoken publicly.

At his death penalty trial on Wednesday, Gargi Datta opened up about her relationship with Holmes, which ended just five months before he opened fire at an Aurora movie theater showing the "Dark Knight" on July 20, 2012. Twelve moviegoers were killed in the mass shooting, while 70 others were injured.

The 27-year-old former neuroscience graduate student is now on trial for a range of charges, including first-degree murder, attempted murder and an explosives offense. If convicted, he will face the death penalty. He, however, has pleaded not guilty by reason of insanity.

While on the witness stand, Datta said she met Holmes in 2011 when they were attending the same graduate school in Colorado. She said their first date was at a horror film festival. They then grew closer as they started hanging out, spending nights together at home watching movies and playing board games with friends. She also described him as a bright but shy guy.

"He wouldn't go up and interact with people," Datta said, adding that Holmes was more talkative when he was alone with her, reports NBC News.

While they were dating, Holmes revealed to her that he had homicidal thoughts, but Datta didn't take him seriously and instead suggested that he seek therapy, reports The Associated Press.

Datta says that she broke up with the mass murderer because she wanted to maintain a more casual relationship.

"I told him that I didn't see a future with him," Datta said Wednesday in her first public comments since the 2012 shooting.

"I had already told him in the start that it was a causal relationship. I didn't feel I was getting closer to him. I think he liked me more than I liked him," she testified.

According to District Attorney George Brauchler, their breakup in February 2012 was a catalyst that triggered him to kill people that summer.

Holmes also confessed that he fell in love with Datta in a video played earlier in the trial.