"He told me he wanted me to make him a naughty old man, or something along those lines," the teenager, who was 15 when she said she messaged with Harris on May 29, 2014, said in a Brunswick, Georgia, courtroom.
The witness said she first lied when she told Harris that she was 18. Harris, who said he was 33, first sent her a picture of his private parts and she responded with a picture of her private parts on Whisper, an app that encourages people to tell secrets, she testified. But later in the conversation, she said she told Harris she was 15.
The girl answered yes and became emotional when Cobb County Assistant District Attorney Chuck Boring asked whether there were "some things in that chat about what he wants to do to you as a 15-year-old girl."
The teenager was one of several prosecution witnesses in Harris' five-week jury trial, including a 22-year-old woman who testified she had a one-night stand with Harris on the sofa of his apartment in 2013. A detective also testified he interviewed a former prostitute who said Harris paid her $125 for a 30-minute session on May 31, 2014, two days after Harris allegedly exchanged the messages with the minor.
Harris, 35, of Marietta, is being tried on charges including malice murder, two counts of felony murder and cruelty to children in the first degree in the death of his 22-month-old son, Cooper, on June 18, 2014. The toddler died from hyperthermia after Harris left him in the hot SUV as he went to work at Home Depot's corporate headquarters in Atlanta, prosecutors said.
Harris also was indicted on eight counts related to his sexual activity with minors, including two counts of sexual exploitation of minors.
Prosecutors say Harris sought sexual fulfillment outside his marriage and had a sordid online presence. He was having illicit chats with as many as six women on the day his son died, prosecutors said. Harris also met with a prostitute in the weeks before Cooper's death, prosecutors said.
Defense attorney H. Maddox Kilgore has conceded that Harris was unfaithful and regularly sexted women, but said his client is not a murderer.
Defense attorneys said Harris did not notice his son was still in the back seat when he went to lunch at 12:45 p.m. on June 18, 2014. Harris' attorneys also successfully argued that extensive publicity may have prejudiced jurors and persuaded Superior Court Judge Mary Staley to move the trial from Cobb County in metro Atlanta to Brunswick in South Georgia, about 300 miles away from Harris' home.
During cross-examination on Monday, Kilgore pointed out that only 21 minutes had passed from the time the 17-year-old started communicating with Harris to the time she sent him a picture of her private parts.
Cobb County Det. Ralph Escamillo testified he found the number of one prostitute Harris met, Daniela Doerr, in Harris' phone.
Escamillo said Doerr met Harris at a Ramada in May 2014, and she performed protected fellatio on him and the two had protected intercourse.
Escamillo said Doerr identified Harris by his booking photo. He told Doerr she would not be arrested for prostitution, he said.
In cross-examination, Kilgore pointed out that Doerr was on probation and could have been arrested for solicitation in a September 2015 sting.
The 22-year-old witness, Jacqueline Robledo, who also met Harris on Whisper, testified that Harris sent her a private message after she posted about seeing the movie "Fifty Shades of Grey."
"It immediately went sexually," said Robledo.
Robledo said she didn't know Harris was married and had a child.
On another messaging app, Kik, the two continued to communicate "back and forth, everyday stuff and also sexual as well," she testified. They met in summer 2013, when she was 19, and had sex in Harris' home, she said.
"I know he was not happy," she told Boring when he asked if Harris had said anything about his family life.
Robledo said Harris also told her about another relationship outside his marriage.
"He mentioned that he was on vacation and that he met up with a guy and that he had relations with him," she testified.
Under cross-examination, she acknowledged that Harris appeared to be in love with his wife and wanted to keep her in his life. He also sent Robledo a picture of Cooper.
"He told you his son was great and he loved his son?" Kilgore asked.
Robledo answered yes.
Prosecutors have alleged that Harris also visited a "child free" message board on Reddit.
During cross-examination, prosecution witness Alex Hall, Harris' co-worker, said he found the child-free website on Reddit and sent it to Harris
"And did you see Ross' response," Kilgore asked.
"Yeah, he wasn't for it," Hall said.
Kilgore continued: "In fact his response was 'grossness.'"
"Yes," Hall said.
Testimony resumes on Tuesday.