Casualty is a British medical drama television series that premiered on 6 September 1986 on BBC1. The series was created by Jeremy Brock and Paul Unwin. It is set in the fictional Holby City Hospital, in the equally fictitious city of Holby, and features occasional crossovers of characters and plots with spin-off show Holby City. Casualty follows the professional and personal lives of the doctors, nurses, paramedics, hospital management and patients at Holby General. It features an ensemble cast of regular characters, and began with ten main characters in its first series, including the longest-serving character Charlie Fairhead (played by Derek Thompson). They have all since left the series. Many main characters have been written in and out of the series since. In addition, Casualty features guest stars each week, as well as recurring guests that take part in story arcs that span a portion of a series or multiple series.

Matthew Afolami, portrayed by Osi Okerafor, first appears in the thirty-fifth series, in an episode originally broadcast on 29 May 2021. The character and Okerafor's casting was announced on 20 May 2021, alongside that of Elinor Lawless in the role of Stevie Nash. Matthew is billed as an experienced locum registrar who has worked in "trouble spots" across the world. The character shares a backstory with established character Fenisha Khatri (Olivia D'Lima); she jilted him on their wedding day, something which is explored in his first episode through flashbacks. Loretta Preece, the show's series producer, praised the relationship between Okerafor and D'Lima. Writers used Matthew's introduction to create tension between Fenisha and her love interest, Ethan Hardy (George Rainsford). Okerafor expressed his joy at joining the cast and looked forward to playing Matthew's devotion to his patients, something which he liked. Preece praised Okerafor for his talent, describing him as "handsome and debonair, sensitive and steadfast, [and] passionate and unpredictable". She added that his "beautifully layered performance" would develop the character further than being a "classic romantic leading man".

Rida Amaan, portrayed by Sarah Seggari, made her debut appearance in the series thirty-seven episode "Welcome to the Warzone", broadcast on 8 April 2023. The character and Seggari's casting was confirmed alongside a new cohort of nurses being introduced to the programme on 23 March 2023.

Paige Allcott, portrayed by Shalisha James-Davis, first appears in the thirty-sixth series, in an episode originally broadcast on 8 January 2022. The character and James-Davis' casting was confirmed on 8 January 2022, just hours prior to transmission of her first episode. Paige is introduced as a bartender, who is joining the medical profession, and arrives in the E.D. after having an accident at the bar she worked at, where she handcuffed herself to a customer to stop him from falling over a railing. Paige introduces herself to the team at the E.D. as the new F1 Doctor, and this is soon established. Paige's storylines have included her relationship with Clinical fellow, Rashid Masum, being tested for the BRCA1 gene, and nearly losing her medical licence after a lack of support from her mentor, Stevie Nash. James-Davies made an unannounced departure on 24 February 2024, in a story arc which saw Paige decide to move to another specialty, and leave the department behind.

Jez Andrews, played by Lloyd Everitt, made his first screen appearance on 12 March 2016. The character and Everitt's casting was announced on 29 January 2016, whilst further details were announced on 24 February 2016 with the casting of David Hide (Jason Durr) and Elle Gardner (Jaye Griffiths). Executive producer Oliver Kent described the three castings as "wonderful" and said their storylines would be "brilliantly vibrant, bold and gripping". Jez is characterised as a "young, ludicrously handsome and very openly bisexual" paramedic.

Cheeky, easy-going and full of banter, Jez is a fresh-faced, newly-qualified paramedic with bags of energy. Jez doesn't take life too seriously, which means he gets on well with everybody and loves a good flirt. It's just a shame that this enthusiasm isn't always applied to his job... Like a distracted puppy, Jez can be difficult to control in a perilous situation, much to Iain's frustration.

— Casualty website (2016)

Kent teased the dynamic between Jez and his paramedic colleague, Iain Dean (Michael Stevenson), calling it "fun" and "unlike anything we've seen before". Tasha Hegarty of Digital Spy described Jez as "Iain's new partner in crime". It was announced that Jez would "fail to impress" Iain with his "cheeky banter and lax attitude" upon his arrival, and would struggle to replace Iain's former colleague, Dixie Dixon (Jane Hazlegrove). Kent also revealed that a "big" friendship would be established between Jez and Max Walker (Jamie Davis). Everitt had already filmed his first scenes when his casting was announced and revealed that his favourite thing to film had been a scene where he rescued a girl from falling from a cliff, "I actually felt it was a real life situation and that excited me." Of his casting, Everitt said, "I feel honoured to be joining such a successful show and look forward to bringing a different essence to a programme that's synonymous with British culture and television. It's actually the first programme I remember watching when I was a child at my Nan's house – there's a great beauty to that."