Spy Gareth Williams was unhappy in London and complained of "friction" at MI6 before he was found dead in a sports holdall, his family have said.
Cycling enthusiast Mr Williams was due to return to the West Country a week after he was found dead because he hated the "rat race" and the "flash car competitions", sister Ceri Subbe said.
She told the inquest into his death she did not believe Mr Williams would let a potential killer in his upmarket London flat, adding: "I cannot emphasise enough his conscientiousness."
MI6 were "dragging their feet" in approving his request in April 2010 to return to GCHQ's Gloucestershire HQ, she added.
Her statement added: "He disliked office culture, post-work drinks, flash car competitions and the rat race. He even spoke of friction in the office."
Ms Subbe also said of her brother: "In terms of a big brother figure, Gareth was perfect."
"It's impossible to do justice to Gareth's impressive character without meeting him."
But "as time went by his enthusiasm began to fade", Ms Subbe said.
MI6 bosses agreed to let him leave on 1 September 2010, but he died only a week before.
The naked and decomposing body of Mr Williams, 31, was found in the bath of his home in Pimlico, west London, on 25 August 2010.
Mr Williams disappearance was noted when he failed to turn up for a meeting at MI6 on 16 August 2010, the inquest heard.
Ms Subbe said she discussed her brother's absence with one of his colleagues.
She said: "He is very conscientious. The person I spoke to agreed, and said Gareth was like a Swiss clock - very punctual, very efficient, and it was very unlike him not to attend a meeting."
Ms Subbe said her brother never told her he was being followed or felt threatened in any way, adding: "I cannot think as to why anybody would want to harm him."