Fans of the show know Gary ended up killing loan shark Rick Neelan in a violent showdown and buried his body in the woods last year.
Shortly after Rick's disappearance, his daughter Kelly became suspicious of the businessman and demanded answers.
Now it has been reported by the Metro that Kelly will return as she is transferred to Weatherfield High.
Gary will soon make a discovery about the money he has been sending Kelly's mum, Claire, for the teen's school fees and ends up confronting Rick's ex-wife.
According to the publication, a soap spy told them: "Gary has tried to put his recent dodgy past behind him and start again with Maria and be a better man - but his past misdeeds have a habit of coming back to haunt him."
"This time it's in the form of Kelly, who becomes quite a presence around Coronation Street - not just for Gary, but when she becomes involved in the Amy, Asha and Summer friendship group.
"Gary will find it hard seeing Kelly around, especially when he discovers what she is going through. As he sets out to support her, she is very much his Achilles heel.
"With Rick's body still out there and Gary's guilt at the forefront, there's a real feeling that Kelly could be his undoing."
According to The Sun, sources say Corrie bosses are thrilled to have Millie back on set and revealed the storyline including Kelly and Gary will be massive.
With Rick's body still out there and Gary's guilt at the forefront, there's a real feeling that Kelly could be his undoing.
The source told the publication: "Millie really impressed bosses when she appeared last year, so she's back.
"The storyline will be huge - Gary's web of lies will unravel when she comes back to the cobbles and tries to find out what happened to her dad.
"She'll be out for revenge if she finds out what Gary did to Rick."
Kelly made a number of appearances last year. When her dad failed to pick her up from school, she went to his office where she met Gary.
Gary pretended to be a client of Rick's, but when Kelly grew suspicious of Gary's behaviour, he told her Rick was a loan shark and had fled the country to get away from police.
Do you want to see Kelly get revenge on Gary?
Coronation Street airs on Mondays, Wednesdays and Fridays at 7.30pm and 8.30pm on ITV.
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