Chester King was the leader of the Kingsman agency. His codename was Arthur.
Background
Recruiting for the new Lancelot
Eggsy, along with several other candidates, are recruited to become Kingsman agents, but during a test in which Eggsy has to shoot his dog JB in order to pass, he fails and Roxy becomes the next Lancelot.
The 'Death' of Harry Hart
Arthur amongst the other agents watch live on a video feed as Harry fights in a corrupt church where, afterwards, Harry is killed by Valentine after killing all the members of the church.
Betrayal and Death
Later, as Arthur and Eggsy toast to the deceased Harry, Eggsy finds out that Arthur had become one of Valentine's converts and Eggsy manages to trick Arthur into drinking the poisoned brandy.