To the rest of the family, he will remain nameless and faceless.
And they will never get to thank him personally for answering their prayers for a miracle.
Only the recipient of the miracle, little Bryan Liu, has the privilege of knowing what his saviour looks like.
In a 20-minute, face-to-face meeting at 3.30pm at the National University Hospital (NUH) on Sunday, the six-year-old boy got to thank the man who gave him a kidney.
Bryan's mother, Madam Serene Ng, 38, said that her son later told her about the meeting.
Madam Ng told The New Paper: "He said, 'Thank you for giving me your kidney. Kor kor (big brother in Hokkien), thank you very much.'
"He also asked the donor how many siblings he has and how old he is. The donor told him that he has two siblings and he is 27 years old.
"But Bryan didn't ask for the donor's name. He called him kor kor instead."
Madam Ng said the donor, who was accompanied by a transplant coordinator, had requested to meet Bryan.
But adults who are related to Bryan were not allowed at the meeting.
Did not see face
She added: "I didn't get to see his face. I only saw his back when he went in on a wheelchair. I saw an average-size guy with short hair."
But for the rest of their lives, the Lius will always remember this stranger as Bryan's guardian angel for his selfless act of giving the boy a second shot at a normal life.
Born with one kidney
Born with one kidney
Bryan was born on Christmas Eve, 2005, one kidney short.
And his single kidney was small and had abnormal tissues. By the time he was two, it had become useless.
Madam Ng gave him one of her kidneys, but it, too, failed and was removed in September 2009.
After that, Bryan had to undergo 10 hours of dialysis at home daily to stay alive while waiting for that elusive kidney transplant.
Day after day since then, the Lius waited for the call that could give their boy a new lease on life.
After almost three years, which must have seemed like an eternity to the family, the phone rang one day in May this year.
It was from the transplant office, and a female voice said: "We have found a donor."
No five words sounded sweeter, and on Saturday, Bryan received a new kidney at the National University Hospital (NUH).
What is rare about Bryan's kidney is that it came from an altruistic living donor - someone who's not related to the family.
Bryan's father, Mr Victor Liu, 50, a telco group manager , said: "We were told that we are not supposed to meet the donor as he's an altruistic donor who is unrelated to us."
Added Madam Ng: "There's protocol to follow."
Mr Liu said: "I can't find words in my vocabulary to express my gratitude. He's a kind soul who made this miracle happen. He went all out just to save my son."
Like Bryan, the donor is recovering from the kidney transplant procedure.
Mr Liu said: "We got word from the doctor that he is doing well. In fact, he's asking how Bryan is doing."
The family got the okay from the NUH's transplant office to pass the mystery donor a hamper through the office on Sunday morning.
Bryan and his twin sister, Charmaine, also made a card for him.
Said Mr Liu: "We put it into the hamper. It is a personalised 'get well' hamper which we did up ourselves with tonics and 'get well' food.
"We wish him speedy recovery, for him to have good health and may all good things come to him in time."
On top of the hamper, the Lius placed a plush toy bear with wings, and on its wrapper, they wrote: "To Bryan's Guardian Angel".
Said Mr Liu: "We will always have the donor in our hearts. The liveliness of Bryan will remind us of this kind soul."
The happy ending had its roots two years ago when The New Paper first wrote about Bryan.
His plight moved 25 readers to pledge to give their kidneys.
It is unclear whether Bryan's angel is one of them.
Madam Ng wonders if the donor is the same person who had sent an e-mail to the Facebook page "A Kidney for Bryan, A Gift of Life" a relative had set up after the TNP report.
In the e-mail, he indicated his intention to donate his kidney to Bryan.
Madam Ng referred him to the NUH transplant office and heard no more about him.
Whoever Bryan's angel is, he had to go through a barrage of blood tests, psychiatric assessments and interviews with the transplant ethics committee to determine if he is a suitable donor.
Professor A. Vathsala, who heads the Adult Renal Transplantation Programme at NUH, said a donor faces two different types of risks.
The most immediate is bleeding around the time of the operation or a blood clot of the legs, which can travel to the lungs.
"This is a real risk and the donor must be informed before the surgery," he said.
For the long term, the donor will have only one kidney for the rest of his life.
If properly screened and found to be healthy before donation, most donors will have normal renal function.
But they need yearly follow-ups to check on their kidney function and protein in the urine as well as other health concerns.
In Bryan's donor's case, Prof Vathsala said the donor was counselled on these risks on several occasions and was given a longer cooling-off period than normal.
"He must be an emotionally strong individual as his only reward in donating would be in knowing that he had donated a kidney to save someone's life," he said.
"He may never have the opportunity to see Bryan have the benefits of the transplant as is often the case for biologically-related donors.
"It is a credit to him that he understood these risks, has acknowledged that he will not gain financially from the donation and has still gone ahead with the altruistic donation."
Mr Liu is aware of the battery of tests the donor had to undergo because he paid for the tests.
He said: "He had gone through the test so many times. After the validity period was over and the process was still not complete, he had to repeat them."
Even after getting the long-awaited call from the transplant office, the Lius tempered their joy with cautious optimism.
Mr Liu said: "We kept our fingers crossed that the donor would be there till the end of the process.
"At that time, we were unsure whether it would go through. There were so many hurdles and he's a young guy."
The family is no stranger to false dawns - potential donors had expressed interest, but couldn't follow through to the end.
"Anyone could be discouraged along the way. Anything can happen along the way," said Mr Liu.
Even when Bryan's angel was going into the operating theatre, he could still change his mind, he said.
"At that point, he can say no and the whole thing can be called off," he said.
But the donor kept to his word and the family's prayers were answered.
Said Mr Liu: "This person is truly admirably remarkable. Now, Bryan has a new lease on life. We will do all we can to keep him that way."
Additional reporting by Judith Tan
This article was first published in The New Paper.