In an unexpected twist due to air this summer, Billy and his partner Julie Perkins (Cathy Murphy) are left stunned as they discover they have a granddaughter they never knew existed.
Although the news is initially a shock, the Walford couple ultimately decide to track down their relative and soon find out that she's living in care - an experience they are all too familiar with themselves.
Billy's granddaughter is 15-year-old Lola Pearce, who has a sharp, cocky but likeable attitude.
As she meets her grandparents for the first time, Lola quickly forms a bond with them as she has been deserted by everyone else in her life.
However, as Lola follows the couple back to Walford, whether the rest of the Square's residents will accept her remains to be seen.
Speaking of her casting, Harold commented: "I am so excited to be joining EastEnders. I have watched the show all my life and I never thought that one day I could be in Albert Square.
"It feels strange to be walking round Walford with people I have grown up watching - I still have to pinch myself."
Viewers will see Lola make her screen debut in the coming weeks.
> Read our EastEnders spoilers and news