She is portrayed by Anna Passey.
Sienna Blake is beautiful, clever and calculating, often keeping secrets from those around her. A troubled girl with serious insecurities, Sienna conceals her cold, cynical and manipulative behaviour beneath a charming, warm exterior. This conflicting personality has allowed Sienna to lure many into her web of lies and deceit.
When an explosion occurs in the local council flats, Sienna kidnaps Tom. She tells him that he caused the explosion and Darren has died. She locks him in the cellar of her childhood home and holds him captive bringing him food daily. Tom is declared missing and a police investigation is launched. Sienna convinces Chloe Chance (Susan Loughnane) to sell her unborn baby to her. Tom manages to escape and Nancy is released from the secure unit. While there, Sienna found her family, Dirk, Patrick, Dennis Savage (Joe Tracini), Maxine Minniver (Nikki Sanderson) and Martha Kane (Carli Norris) kidnapped and held hostage by Will, and discovers that he killed their mother.
She becomes angry and disowns him for what he did. Will is later arrested for his murders of Anna and Texas after Dodger comes to their rescue. After Tom escapes the basement and tells Nancy everything, she confronts Sienna and rips her dress revealing that she is not pregnant. An upset Sienna then tries to convince her brother, Dodger, to run away, but she attempts to kiss him.
Sienna then abducts Tom, Charlie and Oscar and locks them in the Roscoes' garage. She tries to kill them with exhaust fumes, but they are saved and Sienna is sectioned.
It is then revealed that Sienna gave birth to a child when she was a teenager and Patrick told her the baby died but he had actually put the baby up for adoption. Dodger vows to help Sienna find her daughter Sienna does eventually find her daughter, Nico. It is also revealed Sienna and Dodger had slept together when they were younger unaware that they were siblings and their one night stand resulted in Nico's birth. Sienna and Nico have a close mother daughter relationship until Nico accidentally kills Carly, the daughter of Sienna's husband Ben. Nico went on a downward spiral after her murder secret, abusing Sienna and making her miserable.
Patrick had been diagnosed with motor neuron disease and was given about a year to live, and Sienna made up with her father. Soon enough, Patrick sends a video to Ben confessing that Nico killed Carly, but Nico finds out and smothers Patrick to death. Nico's killing spree only continues, killing Sienna's lover Trevor on his wedding day. Ben took the blame for Nico, causing Sienna to divorce him. After a while, Sienna began to form a close relationship with Nico again.
Nico was later caught out for her crimes after Sienna took the blame for Patrick's death and she confessed Nico did it. Sienna tried to convince Nico to hand herself in, but Nico died in a fire when a burning maze killed her, devastating Sienna as she still loved her daughter no matter what.