Cucumber is a non-living cucumber that came free with a purchase of a launchpad from Wal-Mart by Marshmallow, who gave it to Pickle as a gift. Even though the cucumber was obviously non-living, Pickle thought of it as his own girlfriend.
In The Series
In War De Guacamole, the cucumber first appeared when Marshmallow showed Nickel that she got a launch-pad from Wal-Mart, and the cucumber that came with it. Pickle suddenly appears and takes the cucumber caringly, using a pick-up line.
In Sugar Rush, Cucumber appears on a picnic date with Pickle. He continues to treat the non-living cucumber like a living creature, making small talk about what a lovely evening that it is and asking if she was hungry. He claimed to have brought sandwiches, but soon realized he forgot them. Extremely hungry, Pickle jumps at Cucumber and eats it, begging for forgiveness. Marshmallow appears and berates Pickle, saying that she couldn't believe he resulted to cannibalism by eating her gift. Cucumber has, of course, not appeared since.