Cecille is one of two Metal Gear characters based on Konami staff. The first of course, is Metal Gear creator Hideo Kojima himself, who game-wise is part of Mother Base’s staff in Peace Walker, Ground Zeroes and The Phantom Pain.
Cecille is based on his friend, Cecille Camandes, former head of Konami’s french communications office. Although, the character’s middle name-Cosima-is actually a play on Kojima’s name. Together were her last it actually means “Kojima is God”, a bit of an in-joke.
The in-game (Peace Walker) Cecille is a French Ornithologist who was studying birds in Costa Rica-when she stumbled upon the testing of Peace Walker’s Mammal Pod, which had the voice and persona of Big Boss’s/Snake’s mentor, the Boss. She is eventually captured, although the tape winds up in the hands of Professor Galvez and Paz, who use it to convince Big Boss to investigate what’s going on and build up MSF. Paz claims that Cecille was her friend.
Although well-taken care of by Dr. Strangelove, Cecille eventually escapes, and is discovered by Big Boss. She’s then taken to MSF’s mother base via fulton balloon.
At MSF, she denies knowing Paz-who was in fact a CIPHER agent-although this isn’t a big issue until later-and is one of the first signs in the game that Paz isn’t quite what she seems.
Apart from working in the mess hall, she also shares several details about France with Big Boss and Miller, recorded on several tape casettes. She also becomes the object of Miller’s affections, although she mostly finds him annoying, and Miller’s attempts to flirt are a bit clumsy and end up alienating her far more often. In fact, Cecille has eyes more for Snake.
When Strangelove comes to the base, it’s revealed that she actually helped save Cecille from being executed by Hot Coldman, the Peace Sentinel leader who was developing the Peace Walker.
Cecille eventually returns to France as part of preparations in 1975 for the UN inspection, and is spared the destruction of the group and Mother Base. It’s unclear what happens to her in the decades that follow in Metal Gear’s story. Her only appearences in Metal Gear Solid V are in the Ground Zeroes tapes (mentioned by Kaz as being evacuated) and in this photo from the Paz side-ops (Which is pretty much just a photo-shopped version of her Peace Walker character model)