Character witnesses paint a picture of S-21 prison chief as humble, gentle and compassionate.

TWO former students of Tuol Sleng prison chief Kaing Guek Eav, alias Duch, on Tuesday described the former maths teacher as gentle and approachable, and one of his former classmates told the Khmer Rouge tribunal she had been "stunned and terrorised" when she learned of his role at the secret detention facility.

The classmate, Sou Sath, 66, said she had been in a study group with Duch when they were students in Kampong Thom province. She described him as a "humble and docile" person who never hesitated to share his knowledge with peers.

Duch was not particularly outgoing and had few friends, she recalled: "He was a person who was not worried about love affairs."

She said the prison chief offered no hints of his political views, an assertion echoed by Tep Sok, who said he studied under Duch for two years in Kampong Cham in the late 1960s.

"He never said anything regarding the doctrine or the tendency towards communism," said Tep Sok, 61, now a rice farmer in Kampong Cham.

"He always educated us to love human beings and to focus hard on our studies, and that we should be kind to one another and to assist one another when needed."

Another student, Tep Sem, 61, said Duch was a strict, meticulous teacher who nevertheless was friendly with students. "We could chitchat with him, play around with him, and he did not mind," he said.

Asked whether the accused mistreated lazy or disobedient students, Tep Sem said Duch encouraged them to study hard, form study groups and "be attentive to subjects being taught and nothing else".

Tep Sem said Duch was particularly generous with poorer students and hosted extra study sessions for them free of charge. All three witnesses said he avoided conflict with teachers and students.

"In the class he never had any conflict or argument with anyone," said Sou Sath, a retired teacher and former trainer for the rights group Licadho.

Sou Sath said she only learned of Duch's role at Tuol Sleng after reading The Lost Executioner by Nic Dunlop, who discovered the prison chief in Samlaut and interviewed him in 1999.

"I was very stunned and terrorised by reading that book," she said.