"He is one of the movers and shakers. He is very smart, very ruthless, and very ambitious," says one UUP party member about Mr David Burnside, who proposed Saturday's successful motion instructing the party not to participate in the executive until the British government committed itself to retaining the RUC name.
Mr Burnside supported Mr John Taylor in the UUP leadership contest five years ago, but he had no problems working with Mr Trimble whom he described as the "greatest constitutional brain in the party".
However, UUP sources say Mr Burnside became disillusioned with the leadership's policy following the Provisional IRA's refusal to decommission. "He believed a line had to be drawn in the sand and he thought the RUC could be the issue on which to do it," said a source.
Mr Burnside resisted considerable pressure to withdraw his RUC motion from Saturday's meeting. He had withdrawn it at last month's Ulster Unionist Council meeting but stood his ground at the weekend. "He would have been finished in the party if he had capitulated again," said a senior anti-agreement figure.
Mr Trimble is said to be deeply disappointed in Mr Burnside, but the pair remain on speaking terms.
Aged 48, Mr Burnside was born near Ballymoney, Co Antrim. Some UUP sources say Mr Burnside is keen to secure the UUP nomination for North Antrim in the next Westminster election. Mr Burnside denies it. The seat is currently held by the Rev Ian Paisley, but the DUP candidate next time could well be Mr Ian Paisley jnr.
Mr Burnside lives in London but returns home most weekends. Fishing, shooting, and politics are said to be his interests. He has an ambiguous relationship with UUP grassroots.
"On the one hand, he is respected as the local boy made good," says a UUP member. "Yet many people find it difficult to warm to him. He can be quite abrupt. Women in particular find him uncharming."
Reporters complain it is hard to develop a rapport with him. "He prefers to deal with editors," complains one journalist from a British quality newspaper.
Mr Burnside studied politics and ancient history at Queen's University, where he developed an interest in political wheeler-dealing and improving the image of both unionism and Northern Ireland.
"I have always been interested in the marketing of political philosophies," he once said. "If Northern Ireland had the benefit of modern PR techniques, things could be so much better. The unionist tradition has been misrepresented."
In 1975, he was press officer for Mr William Craig's Vanguard Party, a right-wing unionist group. He remained loyal to Mr Craig when other Vanguard members deserted him.
In 1978, Mr Craig sponsored him to spend a year at the Council of Europe in Strasbourg. After that, Mr Burnside became PR officer for the Institute of Directors in London. At his interview, he reportedly admired the portraits of Cromwell and William III and joked: "With those on the walls, you can't fail to give me a job."
It is understood he considered joining Tory Central Office in 1984 before he joined British Airways, where he was director of publicity.
He retains a wide range of toplevel political and business contacts in Britain. He has long been regarded as a moderniser, arguing that the UUP needs better representation and promotion in Brussels and Washington, and that it should take its case to Dublin. He set up Friends of the Union, a high-profile group of British politicians and journalists sympathetic to unionism.