Heidi Costello (Neé Blissett) is a fictional character from the British Channel 4 soap opera Hollyoaks, played by Kim Tiddy. She made her first on-screen appearance on 20 July 2010 and made her final appearance in the series on 1 November 2011, after the character was accidentally murdered by her father, Silas Blissett, in which he mistakes her for Lynsey Nolan (Karen Hassan).
Channel 4's soap opera reporting website Holy Soap describe Heidi as "glamorous and strong-minded". Kris Green of Digital Spy described her as "strong, glamorous and slightly insecure". Speaking of her character, Tiddy stated, "She's an extremely strong woman. She's got a lot of inner strength. She's completely in love with her husband and loyal to her family."
Heidi first appears in Hollyoaks when she views The Dog in the Pond public house with husband Carl. Heidi, Carl and sons Riley and Seth move to Cheste rafter Carl purchases The Dog in the Pond. Heidi fires barmaid Cheryl Brady (Bronagh Waugh) after she flirts with Carl. Heidi sees stepdaughter Jem (Helen Russell-Clark) and persuades her to talk to Carl. Heidi and Carl's daughter Jasmine returns from America and moves to Chester. When Heidi's cousin Mitzeee (Rachel Shenton) arrives they organise a charity fashion show and Charlotte Lau (Amy Yamazaki) offers Heidi the SU bar for the event. Heidi visits Cindy Cunningham's (Stephanie Waring) boutique Cincerity and asks her to showcase her clothes at the fashion show for a challenge. At the fashion show, Heidi wins the challenge. When Mitzeee speaks to Theresa McQueen (Jorgie Porter) through her microphone, Heidi hears that she had sex with Carl. She forgives Carl after he tells her that he had sex with Mitzee before they moved to Chester. She also forgives Mitzeee.
Heidi is strangled to death by none other than her own father, Silas Blissett on Halloween 2011, Silas tells Lynsey Nolan to wear a black cat suit, but other women, Mercedes McQueen, Texas Longford and Heidi wore the suit as well. Silas knocks Heidi out with a brick and chokes her, mistaking her for Lynsey. When Heidi's hair comes out of the wig, he finds out that it's Heidi, and Silas is distraught.
- In October 2011, it was revealed that Kim Tiddy had quit the role of Heidi, with her final scenes airing at the end of that month.