Captain Jonathan S. Tuttle is a fictitious officer created by Hawkeye in the Season 1 episode "Tuttle". So convincingly did Hawkeye, Trapper and Radar maintain the charade that everyone at the 4077th ended up believing that he existed and even felt that they knew him. As part of the joke, the closing credits of the episode listed "Tuttle" as played by "himself".
At the beginning of the episode, Hawkeye and Trapper are loading Sister Theresa's truck with supplies for her orphanage. When she asks who was behind this generosity so that she could thank him, Hawkeye spontaneously creates the name of "Captain Tuttle". He describes Tuttle as a "beautiful man", "George Washington with John Wayne's agent" but a man who was "Captain Humility" and who wouldn't want a fuss.
After Sister Theresa leaves, Hawkeye explains to Trapper and Radar that Tuttle is an imaginary friend that Hawkeye blamed everything for when he was younger, "Who wet the bed?" "Tuttle." "Who broke that window?" "Tuttle." Hawkeye explains that he had no control at all and that when he was drafted, so was Tuttle.
Later, Radar is arranging for more supplies and puts some forms in front of Henry Blake saying he only needs to countersign them because Captain Tuttle, in charge of supplementary supplies, has already okayed them. Henry asks if he knows Tuttle, and Radar convinces him that he must know because he had breakfast with him earlier.
However Henry complicates matters by insisting that Tuttle be rostered as officer of the day. When this appointment is announced on the PA system, Frank and Margaret wonder who Tuttle is and are only narrowly stopped by Radar from seeing the personnel files. That same night, warned by Radar, Hawkeye and Trapper fake in a complete personnel record for Tuttle, full of unverifiable details such as graduating from Berlinisches Polytechnikum. They also insert enough a description of auburn hair and hazel eyes as "a little something for Hot Lips".
The next day, Margaret and Frank finally get to see the file with Henry Blake and they are all suitably impressed. But Margaret wants to check with General Clayton and asks Radar to put a call through. However Radar gets his friend Sparky at headquarters to patch the call back to him so that Hawkeye, from another phone, is able to imitate Clayton and 'confirm' Tuttle's existence.
But things get even more complicated when a finance officer turns up with Tuttle's back pay and insists on giving it to him personally. This time Hawkeye dresses up in a surgical gown and mask and pretends to be Tuttle. He meets the finance office and instructs that the backpay and all future payments should be sent direct to Sister Theresa's orphanage, earning the finance officer's admiration.
Finally the charade almost breaks down when General Clayton hears of Tuttle's generosity and insists on giving Tuttle a commendation. But Hawkeye gets through even this one. During the parade, Hawkeye steps forward to report that Tuttle had died that morning when he jumped from a chopper on a field surgery mission but had forgotten to grab his parachute. Hawkeye then gives an emotional eulogy for Tuttle, saying that "there's a little Tuttle left in all of us. In fact, you might say that all of us together made up Tuttle." So convincingly did Hawkeye do this that Frank and Margaret are almost moved to tears. As a final act of generosity, Hawkeye announces that Tuttle's G.I. insurance will be donated to the orphanage.
In the credits of the episode this can be read "Captain Tuttle as himself"