Easter graduated top of his class from Cambridge University and is a Royal Marine decorated with a Conspicuous Gallantry Cross, awarded for saving three of his men in Bosnia. Also a former SIS officer, he was assigned as the leader of an elite team codenamed JTF-12 comprising of Emily Prentiss, Sean McCallister, Tsia Mosely, and Jeremy Wolff, which was organized by Interpol. The team's job was to track high-profile terrorists, profile them and then go undercover to bring them down. Usually, the country in which they were operating used their own security services to bring them down when enough intelligence had been gathered. Seven years before Season Six, Clyde and his team were charged with tracking and bringing down a former IRA captain named Ian Doyle. Prentiss went undercover as Doyle's lover and he was later arrested. Clyde arranged for Doyle's transport to Camp 22, a North Korean off-the-books prison where Doyle was interrogated for two years. After Doyle escaped from prison, Clyde realized that someone had betrayed the team to him. He soon learned it was Jeremy Wolff and he also begun to suspect Tsia, who was Jeremy's girlfriend, of being in on it.
Prentiss calls Easter and Tsia again so as to find Doyle before he can kill them all. Clyde opposes involving Prentiss' team as they aren't under oath and he believes the BAU won't be able to track Doyle efficiently as even they don't know were he is. Clyde and Tsia later discover that Doyle has come to the U.S. and prepare to track and kill him.
Clyde finds Prentiss sleeping on the metro and, after waking her up, tells her speaking in French, that she is lucky that he isn't working for Doyle. She then describes him her meeting with Doyle and tells him that he isn't working alone. Clyde promises her that he won't let anything happen to her. Later, Prentiss discovers that Doyle had been locked up in Camp 22, a North Korean prison that they kept off the books. She is furious when Clyde tells her that she doesn't need to know about that, resulting in her later telling Tsia that she can't trust Clyde anymore because he knew more than he was letting on and believing that Clyde was the mole who had betrayed them all to Doyle. Tsia is later killed by Doyle, which seems to confirm Clyde's treachery.
By the events of this episode, Clyde is the only one of the old team along with Prentiss to still be alive, McAllister having been killed in Belgium along with his family. He heads to Boston with a case full of cash to try and find Doyle, as he was trying to get information from an Irish mobster, Jack Fahey, with whom he had been in contact with and who had been shot by Prentiss in the ear so that he won't betray her to Doyle. Clyde is arrested by the BAU before he can meet with Fahey. During the interrogation, Hotch, still under the assumption that Clyde is the mole and angry at not getting the information he is seeking, tells him that if anything happens to Prentiss he will destroy him. Meanwhile, the team, with help from Jennifer Jareau, figures out that Jeremy Wolff had betrayed the Interpol team after discovering that he not only was the first killed, but it was done quietly, coupled with the fact that he owned a villa in Spain that nobody seemed aware of its existence, and a lot of money.
Hotch goes back to confront Clyde to tell him that they have found out that it was Wolff who had betrayed them. Clyde then confirms that he had his suspicions and that once in DC he couldn't trust Tsia for that reason. Hotch, knowing that Clyde cares for Prentiss, asks him for his help in stopping Doyle. In answer, Clyde states that the only way to stop Doyle is to kill him and asks Hotch if he will be able to do it. When Hotch says no, Clyde asks him instead to make a promise: to make sure, at any cost, that Prentiss will not be harmed. Clyde finally offers the profile that they had built of Doyle; with that information in hand, the team has little trouble to figure out why he has kept Prentiss for last and finds Doyle's lair. As a result they manage to kill his men but Doyle escapes during the fight, leaving behind a badly injured Prentiss.
Hotch and JJ, knowing that she will never be safe as long as Doyle is at large, help fake her death and send her away with new identities and enough money to be comfortable. Clyde's reaction to these last events is unknown, as he was not seen again. In Hit, he mentions that he didn't attend her funeral because he was "held up in Prague". He also revealed that he somehow knew that Prentiss wasn't really dead and that he had kept tabs on her when she went into hiding.
Clyde reappears in the episode, revealing what happened to him after being interrogated by the BAU. He assists Prentiss in her current case and gives her valuable information about the unsub, all compiled from a case JTF-12 worked on in 2004. He also informs her that he received a promotion. Then, he abruptly offers her a position to run the London office and that the team is hers whenever she wants it.
In Run, Prentiss calls Clyde after an explosion occurred at a bank the unsub was holed up at. He tells her he is sorry to hear about what happened to her team and asks if they are all right. He also asks her again about running office, to which she replies that he has always had bad timing. At the end of the episode, Prentiss is seen looking at her team members mournfully; it is clearly implied that she accepted the job at Interpol. Indeed, from Seasons Eight to Eleven, she has left the BAU for the job.