LOS ANGELES — Kylie Jenner testified Monday that she expressed concerns to her brother Rob Kardashian about his new girlfriend and soon-to-be reality TV co-star Blac Chyna, because she had heard Chyna had a tendency to abuse drugs and alcohol and become violent and had gotten a possible glimpse of it herself.

“I felt it was my duty to express my concerns, but ultimately it was up to him,” Jenner, now 24 and a teenager at the time, said from the witness stand at a Los Angeles trial. She said her brother didn’t heed her warnings, and continued with the relationship.

Chyna is suing Jenner and three other members of the Kardashian family, alleging they defamed her as violent and forced the cancellation of her reality show, “Rob & Chyna,” after one short season.

Both Chyna and Jenner had previous relationships with the rapper Tyga, and Chyna had a child with him. Jenner said Tyga told her dark stories of his time with Chyna and showed her a 6-inch knife scar he said she gave him while abusing drugs and alcohol, Jenner said.

Jenner said that while dating Tyga, she had received at least one threatening text message from Chyna.

“From what I remember she sent me a bunch of devil emojis, and said something like ‘counting down the days’ until she could beat me or something,” she said. “I didn’t report it, because I assumed they were empty threats.”

Jenner testified that when Rob Kardashian and Chyna began dating in January of 2016, she didn’t trust that Chyna’s love was genuine.

“I felt like me dating the father of her child had something to do with it, and maybe she was out with my brother for other reasons?”

Still, she said, she was cautiously optimistic after they announced in April 2016 that they were getting engaged, and said in May that they were having a baby together.

“I was happy for my brother at the time,” Jenner said, “but just curious how it would go.”

“Rob & Chyna” would premiere in September, the couple would have a daughter, Dream, in November, and in December they would have an all-night celebration of their show being approved for a second season that turned into a bitter fight.

That dispute, and how violent it may have become, was at the center of testimony from both Chyna and from Jenner’s mother Kris Jenner. Chyna said she was being playful when she wrapped a phone-charging cord around her fiancé’s neck and grabbed his gun. Kris Jenner, who did not witness the fight, testified through tears that she felt that Chyna had tried to “murder” her son.

Kylie Jenner, whose home the two had been living in, testified that in the aftermath of the fight she saw some of the damage, and remembers her brother being emotionally traumatized, but couldn’t recall seeing any evidence of physical injury.

She definitely came away believing the violence had been serious, she testified.

“He used the words, ‘She was trying to kill me'” Kylie Jenner said.

Jenner returns to the witness stand on Tuesday.

The two other defendants, Khloé Kardashian and Kim Kardashian, are expected to testify later in the week. Kim Kardashian was not in court Monday for the first time in the six-day-old trial.

Another witness, Jeff Olde, who in 2016 was a senior vice president of the E! network, which aired both “Rob & Chyna” and “Keeping Up With the Kardashians,” testified that he approved a second season for the show, though further approval from his bosses would still have been needed.

But his excitement for more episodes was based on the idea that it would include the stars getting married.

“I thought it was really promising because with a wedding that gave us a hook for season two,” said Olde. “Everyone loves a wedding.”

Chyna’s lawyer Lynne Ciani pressed Olde over an email he received from Kylie Jenner expressing concerns about violent behavior from Chyna. The email is a key element of Chyna’s defamation case.

Olde testified that it did not especially influence his decision to reverse course on a second season for “Rob & Chyna.”

“I don’t really take direction from 17-year-olds,” he said.

A similar email soon after from Khloé Kardashian also had little effect, he said. He thought the show should be canceled, he testified, because the couple it featured was no longer a couple.

“It was the Rob and Chyna show and there was no more Rob and Chyna,” he said. “It was not the show we bought.”