William, nicknamed Transfer Billy or simply Billy, is a student who begins attending UCI in Season 3.
Right as Billy introduces himself to Steve in "A Gentleman's Bet", Steve interrupts him and give him a nickname. Later, Frank has Steve convince Billy to have sex with Leslie as part of an annual bet. When Billy says he truly wants Pony, Steve tells him to go for Leslie, claiming it'll get Pony jealous and attract her. In and attempt to get them to copulate, Steve pretends to be a school shooter and is "stopped" by Billy, and they go for their sex. However, Billy sees an ungroomed Pony, who was in a think tank for longer than she desired, causing him to cancel his sex night with Leslie to Steve's bewilderment.
- As shown in "Bi-Topping-Ality", he prefers anchovies over pepperonis as pizza toppings.
- Billy is voiced by Donald Glover, also known as the rapper "Childish Gambino".