Cameron is the floating temp who served as a temporary replacement for Donna as Harvey Specter's assistant. On his first day working for Harvey, he fanned Harvey's folders in alphabetical order and redid his calendar and Donna's filing system, calling it a trainwreck, which frustrated Harvey. He also color codes messages by urgency.
When Harvey finally convinced Donna to work for him again, one of her conditions was that she gets to be the one to break the news to Cameron. She "fires" him, although since he is a floating temp, he merely becomes someone else's assistant.