Bill wears a blue shirt similar to Butters' except with a white collar and pocket. He also wears black pants and matching gloves. His hair is similar to Terrance Mephesto's, although it is brown and slightly shorter. He has thick eyebrows during the first four seasons, but was later redesigned and given regular ones.
Bill is a bully, often laughing at things and people. In addition, he thinks everything is gay. Due to his seeming stupidity, it appears he did not make it to the fourth grade - he is never seen in the fourth grade classroom, but is in the halls, and is only seen in the hallway and playground after the second season. His voice appears to sound almost the same as Fosse's. He was seen again in "Lil' Crime Stoppers" when the boys discovered he, along with Fosse, stole Sarah Peterson's doll. He then played along and was "shot" in the game. Bill seems to be one of the more naive children in the school along with Fosse, Butters, Jason, and DogPoo Petuski. In the episode "It's a Jersey Thing", he was part of Cartman's group to help capture and lock up Kyle. It seems that unlike most other kids, (Craig, Token, Jimmy) who are more aware of Cartman's manipulating methods, he still hasn't wised up to it and so that is why Cartman would most likely seek him out for help.
Due to Bill's quick phasing out from the series, his family was never officially seen. In the episode "Damien", two older men are seen in the bar after Jesus storms out, saying "This show's gay" followed by "Yeah, gay" in the similar style to Bill and Fosse, while also having the same face and clothes but adult bodies. In the commentary of the episode, it was revealed that these two characters were indeed Bill and Fosse's parents. Bill's dad actually has a line in "Ike's Wee Wee" where he is one of the parents that drives by and makes fun of Mr. Mackey. Bill can be seen in the episode "Best Friends Forever" with an adult woman who could be his mother.
- Bill is seen playing on the school's basketball team in "Elementary School Musical", one of the few times he appears without Fosse.
- Bill was a part of Cartman's entourage to eliminate Kyle in "It's a Jersey Thing". Although he didn't speak, it's arguably his first major appearance since "Lil' Crime Stoppers".
- Bill and Fosse are referred to by Matt and Trey within the scripts and commentaries as 'the Nerds', 'the Bullies' and the 'That's Gay Kids' - they actually rarely use the duo's real names.
- Outside of "An Elephant Makes Love to a Pig" he is never seen in the classroom seats with Fosse - Bill is often there into the third season, and Fosse would occasionally reappear. Bill once appeared in 4th Grade Classroom B, but Fosse did not appear with him.
- Bill's voice sounds similar to Kevin McCormick's, and vice versa.
- In "T.M.I.", his name was revealed on the chart as Bill Allen.
- In "Lice Capades" a person looking similar to Bill was seen lining up in the 5th grade line.
- While not a direct relation, Bill is often seen standing with Annie Knitts in background appearances rather than with Fosse, making it one of a few times a character involved in a duo is with another character.
- "诺亚基舟" - He and Fosse make fun of Stan's dog Sparky for being a homosexual.
- "红眼病" - Seen dressed as a Chewbacca.
- "圣诞便便汉基先生" - Participates in the Christmas play.
- "夏天很糟糕" - Seen in the background being chased by Bebe, and playing the tuba in the school band.
- "迷幻天文馆" - Seen in the planetarium.
- "疯狂俱乐部" - Seen in the playground at the beginning playing with an unnamed kid.
- "男孩之战" - Watches the fight between Tweek and Craig.
- "家庭与社会" - Seen duct taping Mark Cotswold to a bench, and at the dance.
- 南方公园加长未删减版 (Cameo)
- "棕色狂想曲" - Seen in the field trip to Arkansas.
- "卡特曼邪教" (Cameo) - Seen in Cartman's church.
- "混沌教授现身" (Cameo) - Seen as a potential fourth friend for Stan Marsh, Kyle Broflovski, and Eric Cartman but does not get picked.
- "狂热天主恋" (Cameo)
- "犯罪克星" - He and Fosse McDonald are the people who stole Sarah Peterson doll.
- "禁烟运动" - Seen running out of the burning school.
- "失恋的斯坦" (Cameo) - Seen playing Football.
- "幻想大陆·上" (Cameo)
- "名单风波" (Cameo)
- "艾滋兄弟" (Cameo)
- "乳腺癌大决斗" (Cameo) - Seen watching Cartman and Wendy fight.
- "歌舞童年" (Cameo)
- "泽西范儿" - Part of Cartman's group to lock up Kyle.
- "厨神兰迪" - Seen in the cafeteria.
- "搞笑机器人" (Cameo)
- "当金潮" - Seen in the background on the playground.
- "霸凌谁之过" - Seen in the background in the cafeteria and later in Stan's anti-bullying video.
- "Black Friday" - Member of the group that wants to get PS4s on Black Friday.
- "A Song of Ass and Fire" - Seen on the PS4 army.
- "Stunning and Brave" (Cameo) - Seen at the student/parent/teacher meeting.
- "The City Part of Town" - In the choir of children welcoming the Whole Foods inspector
- "Skank Hunt" - Bill's girlfriend breaks up with him.
- "Douche and a Danish" - Seen at the battle between the boys and the girls
- "Oh, Jeez" - Seen at the school assembly.
- "Put It Down" - Seen during Cartman's song
- "Hummels & Heroin" - Rallies with Marcus Preston after witnessing his presentation in the school cafeteria.
- "SUPER HARD PCness" (Cameo) - Seen in the gym after evacuation.
- "The Problem with a Poo" - Seen in the school assembly.
- "Tegridy Farms" - Seen playing with Clyde on the playground.
Bill and Fosse are minor characters in the game. They participate in the attack on South Park Elementary and Clyde's fortress alongside the player. Both of them are seen being attacked by three elves, and they befriend the player after he saves them. Bill makes only one Facebook post, and it says "You're gay!"
Bill can be found with Fosse at the Raisins bar. When interacted with, Fosse says, "Sorry, you're not my type" with regards to the player, Bill then responds with, "Yeah, he's taken", referring to Fosse. This could been seen as confirmation that Bill and Fosse are (somewhat ironically) in a relationship with one another; or it may simply be seen as the duo engaging in another example of their "that's gay" routine.Bill Allen is a student at South Park Elementary who is often seen with Fosse McDonald. He is similar to Beavis and Butt-head, describing things as "gay" then laughing in a deep voice.