The focus of the penalty phase in the trial of Boston Marathon bomber Dzhokhar Tsarnaev shifted Wednesday afternoon to the life — and death — of Boston University graduate student Lingzi Lu, the only child in a close Chinese family whose parents reluctantly helped her move to Boston.

The death of the 23-year-old Lu has so devastated her parents that it was impossible for them to travel from China to appear in US District Court and speak on her daughter’s behalf, an aunt of the slain woman testified Wednesday.

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“She loved everything, this girl, in life,’’ said Jinyan Zhao, who has lived in Rhode Island for the past 20 years with her family. “She loved pets. Good food. She tells me how much she likes Boston.”

Tsarnaev was convicted earlier this month of the April 15, 2013, bombing, which killed Lu and two other people, and injured more than 260 others. He was also convicted in the murder of an MIT police officer several days after. In a second phase of the trial, jurors are now deciding whether he should get the death penalty or life without parole.

Zhao, who uses the anglicized name of Helen, helped Lu with her relocation after she arrived in Boston in August 2012 to begin her graduate studies in finance as she pursued her dream of working on Wall Street handling pension funds.

When her parents were summoned to Boston after Lu was killed, they decided to break with Chinese tradition and had their only child buried in the Boston area wearing a tiara and a pink, wedding-style dress that they had once hoped she would wear at her wedding, Zhao said.

“They decided to leave her here,’’ said Zhao. “How she died and why she died — it just felt like she was part of Boston, part of the city. [They] just felt she should be here.”

Zhao said Lu was the family’s only child, a result of the country’s population control rules, and her decision to come to Boston was something her parents struggled to accept. But once they did, they scraped together the money to send her to graduate school.

“I could tell she was really appreciative of what her parents did for her,’’ Zhao said, adding that Lu was the center of a multi-generational family, in which her grandfather taught her Chinese calligraphy and her grandmother braided her hair.

Her grandfather has now crafted a book of memories of Lu, which was shown to the jury.

In an anecdote that brought laughter from jurors, Zhao recalled how Lu was a tiny woman with an appetite so large that she once consumed an entire apple pie, spoonful by spoonful.

“I wondered, ‘Is she going to finish the whole pie?’ And she did,’’ Zhao said. “She was bubbly.”

Zhao said her niece was uninhibited and would jump on a stage and sing without warning and once took a half-hour nap during a test.

“Nobody do that,’’ Zhao said affectionately. “She’s weird.”

With Zhao still on the stand, jurors were shown a videotape of Lu’s father, who spoke at a memorial service for his daughter at Boston University in April 2013. Noting that three generations lived together in China, Jun Lu called his daughter “the family’s little Shirley Temple.”

“Lingzi was never shy. She was full of confidence on the stage, playing piano and dancing,” her father said.

“She is gone, but our memories of her are still alive,’’ said the father, who spoke in Chinese and had his words translated into English. “Lingzi loved all things beautiful. She yearned for life, loved beauty and the pursuit of happiness.When she left for America, she promised to come back.”

He also recalled an ancient saying: Every child is a little Buddha who helps her parents grow up.

He closed his memorial simply. “Lingzi, you are simply the best,’’ he said.

After Zhao’s testimony, the prosecution rested for the day.

Earlier Wednesday, the jurors, who have seen horrifying, graphic photos and videos of the bombing aftermath and heard heartwrenching testimony from people with pieces of metal replacing their legs, were able to smile — when Joseph Rogers talked about his stepson, slain MIT Police Officer Sean A. Collier.

Rogers testified that he married Collier’s mother, Kelly, in 1993 when she was the mother of four children and he was the father of two, a domestic situation he likened to the television sitcom “The Brady Bunch.’’

At the time, Sean was about 6, and the blended marriage brought two Jennifers into the household who became known as “brown-haired Jennifer” and “red-haired Jennifer.”

When Collier’s siblings broke things in the house, Rogers testified, “Sean was always the one to spill the beans,’’ and when brown-haired Jennifer changed into school clothes the parents might not approve of, Collier would tell them.

“He was a cop at an early age,’’ Rogers said.

When Sean played with his brother, Andrew, Sean was always the cop and Andrew was always the bad guy. And when Collier saw somebody pulled over on the side of the road, he would start singing “Bad Boys,” the theme song to the television show “COPS,” Rogers said.

As Rogers told his humorous stories about Collier’s upbringing, many jurors laughed with him. They seemed engaged and looked warmly toward him.

But the sunny memories soon gave way as the father talked about the death of his son at the hands of Tsarnaev and his late older brother, Tamerlan.

Rogers testified that he, his wife, and most of Collier’s siblings went to Massachusetts General Hospital in Boston where Collier had been rushed after he was fatally shot.

“He had a hole in the middle of his head. He was shot to pieces,’’ Rogers really. “They don’t really clean him up that much.”

When his wife reached out to touch her son, her hand came away covered in blood, Rogers said.

Since the murder of her son, Collier’s mother has been diagnosed with post-traumatic stress disorder, in part because Sean was the second child she lost. The first was a baby who lived for just a day or two, Rogers said.

Sean and the baby are now buried together, he said.

“She’s very scared that might happen to her other children,’’ Roger said. “She’s better than she was” but last weekend, on the anniversary of Sean’s death, “she spent the weekend crying.’’

Two people seriously wounded in the bombing, Eric Whalley and Adrianne Haslet-Davis, also testified. Both had been walking with their spouses when two bombs detonated near the finish line of the world-renowned race.

Whalley, 67, is blind in one eye and still has a ball bearing in his head because it’s too dangerous to remove it. The piece of metal impacts his ability to think clearly and has left him and his wife fearful that he is susceptible to a stroke.

“I’m really concerned about a premature decline in cognitive function,’’ said Whalley, whose wife, Anne, was also wounded in the attack. They lost their identification at the blast scene and ended up in two different hospitals, each believing the other was dead.

But their sons tracked down their parents and reunited them in a Boston hospital room.

“I just grabbed her arm and wouldn’t let go,’’ Whalley testified. Whalley was in the hospital for 44 days, his wife for 32. Together they have undergone roughly 45 surgeries.

Haslet-Davis, a ballroom dancer who eventually lost part of her left leg, said she was wearing platform shoes with 4½-inch heels because she was “trying to look cute for my husband,’’ Mark Davis. They were walking near the Forum Restaurant when they heard the first bomb detonate.

“I did not think it was a firework or anything else,’’ she testified. “I knew right off the bat it was a terrorist attack. … I remember grabbing hold of my husband, saying ‘The next one’s going to go.’”

She was right. Moments later both she and her husband became two of the more than 260 wounded by the Boston Marathon bombings.

“I felt nothing. Then I heard my husband scream, scream, earth-shattering. I never heard it before,’’ she said. “I could see him lifting my left leg, my left foot. It was covered with blood. He didn’t stop screaming.”

She said she and her husband were separated by rescuers, and that she found herself in an ambulance being rushed to a hospital, not knowing if her husband was alive. Her parents called her on her cellphone, and she told them that she had been in a terrorist attack.

“I said I’ve been in a terrorist attack at the Boston Marathon, and Adam is dead and this might be it for me,” she recalled.

Haslet-Davis said that since the attack, her husband, an Air Force officer, has sought mental health counseling through the Veterans Administration. Haslet-Davis is now a motivational speaker who still pursues her dream of dancing.

Earlier, Andrew Collier, who is now a mechanic with a NASCAR racing team, testified that it was always clear to his family what Sean Collier’s calling in life was.

“As long as I could remember, he wanted to be a police officer. He would chase us around, making siren noises,’’ Andrew Collier testified. “He would always choose the right thing. Right down to a bug. You can’t kill a bug. You got to put it outside.”

Andrew Collier said that Sean was the one in the blended family of six children who always steered his siblings toward making the right choice.

“Sean was someone that was a moral compass,’’ Andrew Collier testified. “Black and white, what was right and wrong. He was the one always fighting for right.”

He testified that since his brother’s murder, family gatherings are clouded with sadness due to the absence of Sean.

“It’s still a huge loss. Something that will affect me and my family the rest of our lives,’’ Andrew Collier said. “Even when we’re having fun, a cloud.”

MIT Police Chief John DiFava testified that he hired Collier because he understood that on the campus, police have to be part of a unique community composed of “probably the smartest people in the world’’ who require “special attention.’’

“Sean was one of the very best,’’ DiFava said. “He understood people. What made him a police officer was not the uniform, not the badge. It was his character.’’

Since Collier’s murder, DiFava, who once led the Massachusetts State Police, is not so sure he wants his children to follow him into law enforcement. And on the force, he said, there is a continuing sadness for the loss the officers still feel.

“Sometimes I wonder if I want to continue as a police administrator. … I lost one of my own,’’ DiFava said.

Also Wednesday, the jury learned more details about the moment when Tsarnaev made an obscene gesture to law enforcement officials while in a holding cell at the courthouse.

It was July 10, 2013, and Tsarnaev had been brought to the courthouse to be arraigned.

Gary Oliviera, a US Marshals Service employee, said he was monitoring surveillance cameras including the one in Cell 4, where Tsarnaev was placed. He testified that he saw Tsarnaev display his middle finger and that he reported it to supervisors. He was ordered to write a report two days later, he said.

The jury was shown a two-minute snippet of video that showed Tsarnaev before making the gesture. The longer sequence showed Tsarnaev fixing his hair while looking into the reflective camera lens, putting up his index and middle finger in a “V” — and then displaying his middle finger alone.

During cross-examination, defense attorney Miriam Conrad called the gesture a “V sign.” Judge George A. O’Toole Jr. ordered her not to describe Tsarnaev’s actions, ruling that the video speaks for itself.

Tsarnaev’s lawyers, while admitting he took part in the crimes, say he should be spared from the death penalty because he was manipulated by Tamerlan, a dominating, influential older brother, whom they called the mastermind of the attacks. Tamerlan was killed in a violent confrontation with police in Watertown.

Prosecutors say the brothers were equal players. The defense team did not deliver its opening statement Tuesday, deciding to present their statement when they put on their own evidence.

The defense case could start as soon as Monday.

Milton J. Valencia can be reached at Follow him on Twitter @miltonvalencia.Kevin Cullen is a Globe columnist. He can be reached at Follow him on Twitter @GlobeCullen.Patricia Wen can be reached at Follow her on Twitter at @GlobePatty.