Alexandra D'Costa made her first appearance on 17 January 2017. She is portrayed by Sydney Craven.
Alexandra is an evil no good rotten apple of a person and her jail time isn't anywhere near long enough. She attends drama club at Walford High School with Louise Mitchell. She and Madison Drake are kind and caring to Louise. Madison and Alexandra find out that Bex Fowler was involved in sexting so they started to bully her relentlessly.
Madison later "befriended" Bex but it was a ploy to trap her into a prank at her expense. Alexandra and Madison later targeted Louise when she sided with Bex. They spiked Louise's drink in May 2017 until she was coughing up blood and was rushed to hospital.
Alexandra and Madison trapped Bex in a store closet at a Talent Showcase. Their bullying was revealed to everyone when Shakil had turned the soundboard up as Bex had been wearing a microphone, letting everyone in on it. Bex had been rescued by Mr. Pryce, the drama teacher, and her mother, Sonia Jackson.
The head teacher, Mrs. Lund, angrily ordered Alexandra to her office, and an equally angry Louise told her enemy that Bex's mic was on and that the entire auditorium had heard everything. For this, Alexandra and Madison had been put into external isolation and were banned from attending the school prom.
A furious Alexandra schemed with Keegan Baker to get into the prom. At this point, Madison did not realize what Alexandra was up to.
At the prom, Alexandra viciously pushed Louise during a row and she fell back on some candles. Her dress caught fire and the sprinklers went off when the smoke alarm was activated. Louise would later be revealed to have had serious third degree burns on her back and would have to have skin grafts.
Keegan was so furious that he dragged Alexandra onto the stage in front of everyone and exposed what she had done to Louise.
While talking with the police, Alexandra tried to shift blame onto Madison for what happened, which failed. The two girls nearly came to blows, ending their friendship for good. Madison told the truth of what happened, which showed that she had some remorse, unlike Alexandra, who was proud of what she did. She was so nasty that she would have lied to make sure Madison would have gotten the full blame for it.
Madison had not realized that Alexandra had done this, yet, the two of them were both arrested, leading to both characters departing from the show.
As the two were marched out by the police, a defiant Alexandra and a tearful (and remorseful) Madison were deservedly berated by Louise's classmates for the viciousness they had seen.
Mr. Pryce revealed to Sharon Mitchell that the two girls would be prosecuted as adults for what they did to Louise.
It would be later revealed by Sharon Mitchell that both Madison and Alexandra were expelled from Walford High due to their antics. They parted ways with Louise and Bex for good and were never seen in Walford again.