Andi, I am not your sister. I'm your mother.
— Bex to Andi
Bex is the daughter of Celia and Ham Mack. For most of her life, she has lived an adventurous life, travelling the world. While still a teenager, she got pregnant and gave birth to a baby girl, Andi Mack. After a disagreement with her mother Celia, Bex left the baby Andi under Celia and Ham who raised her as their own daughter. Bex continue to travel the world and whenever she came home, she as well as her parents told Andi that she was Andi's older sister.
On Andi's 13th birthday, Bex comes back home ready to finally settle down. Andi is excited to have her cool older sister home for her birthday but Celia is not impressed. After hanging out with Andi on her birthday, Bex feels compelled to tell Andi the truth: that she is not her sister; she's her mother. She shows Andi a picture from the hospital bed with Bex holding Andi after she was born.
After exposing the secret, Bex is currently trying to make things right between her parents and her daughter. In Shhh!, she has gets herself a job at The Fringe where things are a little awkward because her boss is Brittany, a girl she used to babysit.
Bex has been struggling to be the perfect mother for Andi while at the same time acting like Andi's cool sister. This becomes a problem in Shhh! when Bex encourages Andi to stay up late watching a horror movie, against Celia's wishes. As a result, Andi gets detention the following day for sleeping in class. It gets worse in Dancing in the Dark when Bex asks Andi to throw a dance party while the parents are away. Not only does this give a room for Amber to humiliate Bex and Andi, but it also upsets Celia so much that she gives them a silent treatment. By It's Not About You, Celia is still not talking to them and Andi starts avoiding Bex, because she feels like Bex is a bad influence on her. Meanwhile, Bex has been trying to figure out a way to talk to reach out to Andi's dad.
In She Said, She Said, Bex tries to prove that she's responsible to Celia by buying groceries and cleaning up the house but she finds it difficult to get approval from Celia. It gets awkward when Celia visits Bex at The Fringe and realizes that Bex's boss is Brittany since Celia and Brittany have some bad history. Celia asks Bex to give her a makeover to go see Wicked. Bex is proud when Celia says she is good in makeup but it doesn't last long before Celia brings up the keyboard that Bex destroyed. Later on, Bex tries to help Andi miss school by giving her tips on how to fake being sick and ultimately offers to write a note for Andi.
While fighting with Celia on whether Andi should go to school, Bex is shocked when Bowie Quinn arrives. She introduces Bowie and Andi and tells Andi that Bowie is her dad. In Dad Influence, Bex asks for Andi's permission from school so that she could spend time with Bowie. At the end of the day, Andi wonders why Bex broke up with Bowie but Bex explains that it's partly because he's unpredictable. Bex and Andi find Bowie in Andi Shack, having invited himself to be their guest. Together, they convince Celia to allow him to stay for a while.
In Terms of Embarrassment, it's revealed that it was Bowie who walked out on Bex after a fight. He says that he always wanted to come back to Bex once he made it in life but it never happened. He apologizes to Bex and asks if she could let him back in their life but Bex isn't ready so she avoids the talk.
After Bowie leaving, Bex tries to connect more with Andi in She's Turning Into You. She takes Andi to The Fringe for a shopping spree where they try on different outfits for fun. She tries to play a match-maker for Andi and Jonah when Andi brings Jonah to The Fringe to shop for Amber's birthday gift. Bex puts make-up on Andi infront of Jonah to show him how pretty she is. Bex takes Andi to The Spoon Diner to eat baby taters and talk about Jonah Beck. Andi has so much fun that she wonders why Bex even left in the first place - they could have been having all that fun every day. Bex is ready to explain how she left but Andi just wants to talk about Jonah. The two arrive late for dinner, making Celia really mad. Bex and Celia have a very heated drama during which they debate the circumstances under which Bex left in the first place. Celia blames Bex for leaving while Bex believes that Celia sent her away. Andi runs away from the heated drama. The following day, she puts on heavy make-up before leaving for school. Celia gets scared that Andi is turning into Bex.
Following the intense drama, Bex doesn't want to stay at home with Celia. So, she gets an apartment and takes Andi there after picking her up from school. Life in the new apartment turns out to be more difficult than expected in Home Away From Home. Bex struggles to keep things running in the apartment: taking care of Andi; doing laundry and providing food in an apartment where electricity barely works. She helps Andi sneak back into the Andi Shack to get art supplies but they get busted by Celia and Ham. Following this and after realizing that Andi she is not able to fully provide for Andi, she returns her to live with Celia and Ham. However, Andi still chooses to live with Bex because it's time to embrace the new times. Bex is filled with Joy when Andi calls her "mom" for the first time.
In Were We Ever?, Bex mobilizes The Good Hair Crew to lead a protest against unreasonable dress code at Jefferson Middle School. She supplies students with prison outfits for the protest, but runs into Principal Metcalf. After being called into the Principal's office, Bex and Andi manage to convince the principal.
Bex gets a pleasant surprise when Bowie returns to town in Best Surprise Ever. Andi gets Bowie to play the song he had written for Bex. This puts Bex and Bowie in a good mood and they dance together, just like the old times. But after that, Bex lets Bowie leave. She tells Andi that she still loves Bowie but they're not getting back together. She makes it clear that Bowie is not someone they can fully count on to be there when they need him.
Bex is fun and adventurous. She travels the world and rides a motorbike. Andi admires Bex because of that and she thinks Bex is cool.
- Andi has always looked up to Bex as her cool fun sister. She always looked forward to Bex coming home so they could spend time together. She tried to hook up Andi with Jonah Beck. Even after revealing that she's Andi's mother, Bex still continues to act like a sister to Andi.
- Bex and her mother, Celia don't seem to be in good terms, probably because she abandoned the family and went to travel the world. It doesn't help that Bex tells Andi the truth, forcing Celia to come to terms with being a grandmother.
|Celia :||Oh Bex, I wish you'd just grow up already.|
|Bex :||I have! This is it! I am here. And I'm raising my daughter. I have a job, that I like, that I'm actually good at. Believe me, I live for the day you tell me you're proud of me. But it's not gonna be today, is it?|
|— Bex to Celia, "She Said, She Said"|
- She was a teen mom.
- She is the actual mother of Andi, not her sister.
- She rides a motorcyle.
- Bex doesn't act like a mother to Andi.
- Bex really likes that Andi calls her "mom".
- Her and Andi moved to a new home.
- She has a box fulled with stuff from her past.
- Bex never told Bowie that she gave birth to her daughter, Andi.
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