Neighbours is an Australian television soap opera created by Reg Watson. It was first broadcast on 18 March 1985. The following is a list of characters that first appeared in the serial in 1990, by order of first appearance. All characters were introduced by the show's executive producer Don Battye. The sixth season of Neighbours began airing from 11 January 1990. January saw the arrival of the Alessi twins Caroline and Christina Alessi played by Gayle & Gillian Blakeney. Barman Kelvin Stubbs also began appearing that month. In February, Josh Anderson and Ryan McLachlan, played by Jeremy Angerson and Richard Norton, respectively, made their first appearances. That same month British actor Derek Nimmo guested as Lord Ledgerwood. Maggie Dence arrived as new principal Dorothy Burke in March. Bob La Castra joined the serial as Eddie Buckingham in April, and Stephen Hall also joined the cast that month as Boof. Beth Buchanan began playing Gemma Ramsay, the niece of established character Madge Bishop in June. The following months, three more members of the new Willis family arrived, Doug, Pam and Adam, played by Terence Donovan, Sue Jones and Ian Williams, respectively. Alison Whyte guested as environmentalist Amber Martin in September and Richard Huggett arrived as Glen Donnelly.

Caroline Alessi, played by Gillian Blakeney, made her first on-screen appearance on 18 January 1990. Writers devised relationship stories for Caroline alongside the characters Adam Willis (Ian Williams) and Jim Robinson (Alan Dale). A notable storyline for the character was kissing her sister Christina's (Gayle Blakeney) husband Paul Robinson (Stefan Dennis).

Christina Alessi, played by Gayle Blakeney, made her first on-screen appearance on 18 January 1990. Christina is the twin sister of Caroline Alessi (Gillian Blakeney). Christina later marries long-standing character A notable storyline for the character occurred when she ran off with her son Andrew Robinson (Shannon Holmes) without informing Paul.

Kelvin Stubbs, played by Michael Fletcher made his first on-screen appearance during the episode broadcast on 22 January 1990. Fletcher is a British actor and had appeared in Neighbours four times prior in small "walk on" roles as delivery men. Kelvin is portrayed as a "bashful" and "clumsy" character and Fletcher believed that Kelvin has "bad luck". Producers asked Fletcher if he was clumsy like Kelvin and he insisted he was not. Fletcher told that Stuart Gilles from Manchester Evening News that he had an ironic first day filming because he had a series of clumsy accidents.

Writers paired Kelvin romantically with Melanie Pearson (Lucinda Cowden). Fletcher recalled that writers transformed the character's appearance and persona, which leads to romance with Melanie. Fletcher told Gilles that Kelvin "always fancied" Melanie but believed she would not want a "bashful type" as a romantic partner. Melanie later pursues romance with Kelvin when he changes his appearance. Fletcher revealed that Kelvin becomes "less clumsy" and "transformed into someone "very different" but not "entirely macho". He revealed that he was required to wear glasses to make him more attractive to Melanie, despite not wearing glasses off-screen. Writers opted for a passionate relationship for the duo, with more intimate scenes being written than were broadcast. Fletcher revealed that he and Cowden changed some intimate scenes they believed were "over the top" because they were always kissing. He described their relationship as problematic because "they keep falling in and out of love". Fletcher described the end of Melanie and Kelvin's relationship as "surprising" and recalled crying when he first read the scripts.

Kelvin is a waiter at Lassiter's. He is revealed as a secret admirer of Melanie and she accuses him of stalking her. She decides to help him gain confidence and they begin to date. Melanie gives Kelvin too much confidence and he asks Christina Alessi (Gayle Blakeney) out to dinner and goes on a date with Andrea (Bridie Carter). Melanie is upset to realise Kelvin has been cheating on her. He made his final appearance as Kelvin on 3 April 1990.

Joshua "Josh" Anderson, played by Jeremy Angerson, made his first on-screen appearance during the episode broadcast on 5 February 1990. He arrives in Erinsborough because his father relocates to the area to run a news agents. Producers hired Angerson because they were increasing the size of the male cast following the departure of popular characters. Josh is characterised as friendly, academically bright and good at chemistry. The character helped to form the show's teenage character group of his era alongside Todd Landers (Kristian Schmid), Melissa Jarrett (Jade Amenta) and Cody Willis (Amelia Frid).

Ryan McLachlan, played by Richard Norton, made his first screen appearance on 15 February 1990. Norton told Chrissie Camp of TV Week that he "couldn't believe his luck" when he received the role of Ryan. He moved to Melbourne aged 19 to begin filming. Ryan is the older brother of Tiffany McLachlan (Amber Kilpatrick) and very protective towards his younger sister. Ryan and Tiffany come to Ramsay Street to stay with their aunt, Dorothy Burke (Maggie Dence). Norton stayed with Neighbours for one year and left to join the cast of rival soap opera Home and Away. Writers devised an exit storyline which saw him leave Erinsborough to join the army.