William "Billy" Jessup is the secondary antagonist of Jumanji. He was the ex-boyfriend of Sarah Whittle when they were teenagers, while he was also an enemy of Alan Parrish due to jealousy of the positive friendship between them and possibly Alan's superior heritage and living class. He was the catalyst for Sam Parrish's meaningful lesson about standing up to face his fears.
Billy Jessup was half 13 year old, half attack dog and had been hard on Alan for a long time due to hanging out with his girlfriend, even though Sarah told Billy to stop being so aggressive. While Alan was riding his bike minding his own business, Billy hid behind the town statue of General Angus Parrish and pursued Alan with his gang, threatening his life. Alan had one advantage over Billy's gang, their bikes were inferior to his but Billy hung around outside the Parrish Shoe Company's Factory and waited for Alan to come back out, where he and his gang continued to beat him up and stole his bike.
Billy is not seen again for the rest of the film or the novel but it remains clear that his relationship with Sarah did not continue in both Jumanji's alternate timeline and the restored timeline, Sarah even seemed to have forgotten who Billy even was beyond him being the kid that picked on Alan.