Tweek's jitters were caused by excess coffee consumption, which occurred because his parents own a coffee shop that almost went out of business in the episode entitled Gnomes.
Tweek fought with Craig in episode 304, entitled Tweek vs. Craig. He was chosen as the fourth friend after Kyle, Stan, and Cartman kicked Butters out of the group.
Whenever Tweek is told to do something, he responds by saying, "Aaagh! Way too much pressure!" Tweek seemed to practice fatalism in episode 511 (Osama bin Laden Has Farty Pants)
Tweek has blonde hair that looks as though he rolled out of bed, wears a gray shirt buttoned wrong, and gray pants.