Neil was a new student to Greendale, hoping for a clean slate after having been saddled with the nickname Fat Neil his entire life…but the nickname followed him.
In order to make him feel better, Jeff feigned interest in Neil's passion, Dungeons and Dragons. Neil brought Jeff all his D&D books one day claiming to have no further use of them. Annie saw this as pre-suicidal behavior so she and Jeff called the rest of the group together to hatch a plot to help Neil reclaim his confidence.
As the game commenced, Pierce crashed the game and became the villain. The group renewed their vigor to take Pierce out, but he gathered information on D&D from another student and figured out how to take control of the game, leading to the reveal that Jeff was the one who coined the nickname "Fat Neil."
After Jeff's apologies, Neil led the group to victory, taking Pierce down.