Cassiopeia Williams

"'I lied before. My name is Cassiopeia Williams - Cassie for short. You recognise me because my sister was Mercury. I'm so sorry.'"Dr Cassiopeia "Cassie" Juliet Williams M.D. is the sister of Mercury Williams † and girlfriend of the unseen Freddy Lutz. She is a training intern who has expressed interest in the cardiothoracics section.

History
Cassie was born in Stockholm a year after her sister to James and Olivia-Ebba Williams. The 25 year old went to med school and graduated before going to work at London Hospital alongside Dr Clary Fletcher, a woman whom she greatly admired.

Memory of Mercury
Her memory of Mercury comes back to haunt her when Liam Stanton turns up in hospital. Clary tells her not to see him, but she cannot stay away. Two hours before he's discharged, Cassie tells him who she is. Arguably, she says it in such a way that Liam is rendered speechless, but in the end she gets her words out and apologises for and inconveniences and goes to leave, however Liam invites her out for a drink despite her being in a relationship. She tells Liam how she felt when Mercury died and that he should live his life because she can see how unhappy he is. Spurred on by this, he goes to ask Clary out. Cassie, now remorseful of her words, is anxious about their relationship, but Clary tells her to "get a grip" and that "it's the right thing". Eventually, Cassie ends up being happy for them. She also says that she shows no interest in Liam as it would be immoral to her sister's ghost.

Tower Bridge Fire
During the Tower Bridge Fire in Series 4, she aids her attending, Clary, in saving many victims, including Dominic Stanton’s wife, Josephine Shelby. Later, she is seen to be hugging Liam who tells her she’s got the same fight and fierce nature as her sister.

Mercury's Birthday
During Series 5, Mercury would've turned 26, in Episode 5.6. Cassie is seen to be in a pub in London, near Coventry, where she is drinking down her sorrows while Liam sends balloons up to the heavens. She was excused from work due to her mourning.

Leaving London
Cassie finishes her internship and departs from London Hospital as she has a placement in Edinburgh. She says her goodbyes to Clary, Liam and Josephie (whom she helped save). She departs, with 5.8 being the last time Cassie is seen as a main character in the show.

Clary and Liam's Wedding
After receiving a mysterious invitation from Josephine in Series 6, she appears at Liam and Clary's wedding, much to their shock and surprise, but happiness. She says that Liam's to do what's best for him, with Liam confessing that maybe Mercury wasn't the love of his life, Clary was. Cassie says that if Mercury was alive, they may not even be together. Liam ponders that but doesn't allow her to elaborate before Clary pulls him off to dance.

Freddy Lutz
Cassie was dating Freddy Lutz through the duration of Series 4 and prior to that. Judging by her body language towards Liam, had she not been dating Lutz, she may have tried to make advances towards him.

Trivia

 * Suki Waterhouse was the first choice for Cassie.
 * Cassiopeia is a constellation while Mercury is a planet. Both Elsa and Juliet are Swedish classic girl names.
 * Cassie was supposed to originally leave the show after Series 4, but Suki’s portrayal as Cassie got such a good reception that the producers renewed Waterhouse’s contract for another series. At the end of Series 5, Suki wanted to leave. In the “New York Times” she called it the “right time to leave. Cassie wasn’t supposed to be more than a filler for Liam’s sadness. Now he’s over Mercury, she can go.” However, Waterhouse did agree to appear in 6.10.
 * Suki, who is a huge fan of "Grey's Anatomy" said she wanted her character's speciality and interest to be in Cardiothoracic surgery as her favourite character on the show was Cristina Yang.