The phrase "a key figure when" is correct and usable in written English.
It can be used to indicate someone who plays an important role during a specific time or event.
Example: "She was a key figure when the organization was founded, helping to shape its mission and vision."
Alternatives: "an important player during" or "a significant contributor when".
It is a theory sniffily dismissed by Haye's trainer and strategist, Adam Booth, who could be the key figure when the lippy Londoner finally climbs into the ring against the erudite Ukrainian Wladimir Klitschko before a 50,000 crowd at Hamburg FC's Imtech Arena next Saturday.
When a key figure in a company sells shares, it's hard to blame other investors when they follow suit.
And when a key figure in the school is diagnosed with a social disease, guess who is blamed?
Speculation over whether Christie knew about the closures intensified on Friday, when a key figure in the controversy alleged that the Republican governor had lied to the public.
But the highlight of the day came when a key figure from the first season of Serial, Asia McClain, offered her account that she had seen Syed at a Woodlawn library during the time when prosecutors said he had killed his former girlfriend.
Set in a Midwestern metropolis that grew up around a refinery, his muscular novel initially takes a long view of the cynical bargain struck between civic leaders and organized crime — and only moves in for the kill when a key figure in this devil's dance decides to reform.
He was a key figure in starting the myth; when he attempted it, other southern leaders called him on it.
Braun was a key figure in the Brewers' 2011 campaign, when the team reached the National League Championship series but were beaten by the eventual MLB champions, the St Louis Cardinals.
Rooney will be a key figure at Hull on Boxing Day, when United look to extend their winning run to five matches, which would be the most they have managed under Moyes so far.
