Cheese is an NPC in Fusionfall: Legacy. His home series is Foster's Home for Imaginary Friends.
Cheese is an Imaginary Friend created my Mac's next door neighbor Louise. Funnily enough, however, when Mac first met Cheese, he believed the Friend to be one that he created. Luckily, Louise eventually showed up to correct this mistake, something Bloo was more than happy about, since he, along with plenty of others who lived in Foster's Home for Imaginary Friends, thought that Cheese was extremely annoying. Unfortunately for them, Cheese would continuously appear at Foster's several times: he became the reason Mr. Herriman tried installing a security system in "The Big Cheese", he followed the gang to the movies in "Cheese A Go-Go" (in which Bloo ended up thinking Cheese was an alien), and he eventually even became a resident of Foster's in the series finale "Goodbye to Bloo" (much to the annoyance of almost the entire cast).
In Fusionfall, Cheese appeared in Hero's Hollow in the past, but was never present in the future, hinting that he was either captured or killed by Fuse and his minions. Whether that will stay the same in Legacy is currently anyone's guess.