[2] Zack is initially introduced as the estranged son of Gavin Sullivan (Paul Nicholas) and the half-brother of Sharon Watts (Letitia Dean).

Zack’s storylines have included a tumultuous relationship with Nancy Carter (Maddy Hill) and a feud with her father Mick (Danny Dyer), a fling with Sam Mitchell (Kim Medcalf), learning that he has HIV,[3] losing daughter Peach with Whitney Dean (Shona McGarty),[4] and then deciding to foster after struggling with the initial thought of raising a child that is not his own.

[5] He later takes on legal responsibility for his on-screen nephew Albie Watts (Arthur Gentleman) and Britney Wainwright (Lola Campbell) - albeit illegally.

[6] Zack tells Sharon about a time when he baked Gavin some cakes at school but he was unappreciative and stubbed out his cigarette in one, leaving him humiliated.

As he goes to leave, Sharon tells him that Dennis died the previous year and asks Zack to stay, to which he initially refuses, although returns later with food and wine for them both.

Zack finds himself on the wrong side of Martin Fowler (James Bye), who punches him when he believes that he has been sleeping with his wife Ruby Allen (Louisa Lytton).

Zack starts dating Nancy Carter (Maddy Hill) and also flirts with her half-sister Frankie Lewis (Rose Ayling-Ellis).

Zack persuades Frankie to cover up the accident but it proves too daunting for her and she confesses to Nancy's father Mick (Danny Dyer).

Whilst upstairs in The Queen Vic, Nancy's mother Linda (Kellie Bright) is threatened by burglars who attempt to steal money, but Zack stands up to them and they leave.

Zack builds his relationship with Nancy, and she supports him when he has the opportunity to participate in a cooking competition, letting him use The Queen Vic's kitchen for practice.

He supports Nancy when she learns that her aunt Tina Carter (Luisa Bradshaw-White) – who is thought to have been on the run – has been murdered by Gray Atkins (Toby-Alexander Smith).

Nancy finds out and after much consideration, she decides to make a fresh start by leaving Walford without Zack, thus ending their relationship.

Zack's former friend, Brett Nelson (Fabrizio Santino), arrives in Walford and applies to become a personal trainer at Sharon's gym.

After messing up an opportunity to become a head chef at Walford East, he sees Brett who encourages him to get checked at a sexual health clinic.

Not long after, Whitney discovers that she is pregnant again and decides to keep this from her adoptive aunt Sonia Fowler (Natalie Cassidy) who is also trying for a baby but had not been unsuccessful.

Bianca rescues the pair and Whitney gives birth to Dolly Dean-Hudson but at the hospital, Britney overhears Zack and Lauren talking about their one night stand.

Whitney is aware that she is being gossiped about and she refuses to reconcile with Zack, calls off the wedding and ends her long-standing friendship with Lauren.

Whitney then realises that she has wasted her whole life relying on a man and plans to make a fresh start as a single mum to Britney and Dolly by moving to Wakefield.

He later discovers that Denzel Danes (Jaden Ladega) has been taking steroids and has provided them to Ravi's son Davinder "Nugget" Gulati (Juhaim Rasul Choudhury), who overdoses.

Zack is introduced as the son of Gavin Sullivan (Paul Nicholas) and half-brother of established character Sharon Watts (Letitia Dean), although this is embargoed until his first appearance.

[13] The storyline begins when a face from Zack's past, Brett Nelson, played by former Hollyoaks actor Fabrizio Santino, arrives in Walford in search of employment at the local gym.

[14] On the storyline, executive producer Chris Clenshaw addressed, "EastEnders has never shied away from covering sensitive issues and Zack's story is one of these.

[7][8] The storyline also covers HIV and pregnancy, as Zack is diagnosed with the virus shortly after impregnating Whitney Dean (Shona McGarty).

On this, Dr. Kate Nembiar from Terrence Higgins Trust added, "All pregnant people are offered an HIV test to reduce the number of babies born with the virus.

[7][4][8][3] Zack and Whitney's relationship has proved popular with fans, though Shona McGarty quit the soap opera in 2023 to pursue other projects.