Dr Teo said he had met fellow brain surgeon Suresh Nair in 2000, when the latter was a registrar at Prince of Wales Hospital.
Although describing Nair as "a bit odd" and "awkward socially", Dr Teo said the pair had spent up to 16 hours a day working together and become close.
"I spent so many hours with him, I got to know the real Suresh in that year... I had nothing but good things to say about him," Dr Teo told the District Court.
"(He is) a fine person, a fine character with a good heart ... (He had) a peaceful sort of soul who cared very much for his patients and I thought he was an excellent neurosurgeon."
Dr Teo said he knew nothing of Nair's cocaine addiction and was shocked when Nair was initially charged with the manslaughter of one escort Victoria McIntyre in February 2009, and the murder of call girl Brazilian national Suellen Domingues Zaupa nine months later.
He has ultimately pleaded guilty to the manslaughter of Ms Zaupa, and one count of drug supply relating to Ms McIntyre's death.
Nair, 42, also pleaded guilty to a charge of supplying a prohibited drug while on bail last January.
A preliminary hearing of the case revealed Nair's penchant for having "model type" escorts visit his apartment to consume large amounts of cocaine, sometimes in "rock" form he inserted into their bodies.
There are many surgeons who, because of ego, would rather let their patients die than refer them to someone else," Dr Teo said.
"But he would refer patients to me regularly that he thought I could help with."
Dr Teo said that he had initially found Nair to have an "odd personality" when they started working together at Prince of Wales in 2001.
But over the course of many hours spent together the two had formed a very close bond, he said.
"I don't believe that Suresh would ever operate on a patient under the influence of drugs.
"His patient care was of the highest level.
"Patients speak very highly of him."
Dr Teo said that his family and colleagues had not wanted him to "stick his neck out" by supporting Nair but that he had wanted to support his former colleague and friend.