Sir James Mitchell is a British spymaster.
He performs this onerous task during the incredibly stressful Second World War where getting the intelligence the various government agencies and military services so urgently need is the job for highly trained and dedicated men and women. It is Mitchell who has been tasked with leading one such group of operatives and it likely he who decided to hide the identity of those personnel using cover-names of flowers as their monikers.
"To protect their identity British agents operated under a code name ... the name of a flower". Those are the words used in our introduction to the organization that Mitchell created and heads. We are shortly thereafter allowed to visit "an ordinary-looking, but hush-hush house 'somewhere south of London'". "This innocent-seeming country mansion was the headquarters of [Mitchell] who under the cover name 'Carnation' controlled a group of super-agents, everyone called after a flower ... and deadly flowers they were!"
Thus we learn that Mitchell/Carnation is the head of the 'Agents of the Flowers', a name which does not quite have the gravitas that one might expect from such deadly and dangerous individuals. Mitchell takes the use of these cover-names quite seriously; we watch in the first recorded adventure him dress down Agent Poppy, "You never mention real names. You may be Colin Ransom to your girl friend and your mother. To us, you're just Number 17. In addition, [you will be given] your special codenames... We're dealing in flowers". In that lecture, Mitchell reminds the agent of another operative, Number 15, who slipped up and used a real name and was overheard and died as a result. Mitchell knows the dangers which is why he is such a stickler.
During that visit where we meet Mitchell, we see that he is a man likely in his late fifties, sporting tightly cut hair brushed back revealing a receding hairline. His face is still strong and determined, revealing a man who has served in harsh conditions for many years and lived to fight yet another day. Due to his age, however, he is no longer the right man for the field and so he uses his experience and his knowledge by training and then selecting the men who will head out into danger. Dressed religiously in tweed suitcoats and with a pipe seemingly affixed to his mouth, Mitchell makes the hard calls as to who to send on the next mission, well aware that that individual might never return.