McClincy's casting comes weeks after it was learned Samantha Morton (Minority Report, Fantastic Beasts and Where to Find Them) will play Alpha, who in the comics emerges as the series' primary human threat following the defeat of ousted Savior leader Negan.
In the books, the 16-year-old Lydia enters into a Romeo and Juliet-like romantic relationship with Carl as the two are separated by their respective warring factions of survivors. Carl belongs to the Rick Grimes-led Alexandria, and Lydia to the Whisperers: a savage, unforgiving pack of nomads clad in walker skins, who use the deceptive practice to both disguise themselves and move freely amongst the dead.
The enemy group was finally confirmed by The Walking Dead Season Nine San Diego Comic-Con trailer, seen pursuing a mud-covered Rosita (Christian Serratos) and Eugene (Josh McDermitt) as the "walkers" — actually the Whisperers — hiss, "Where are they?"
Carl's live-action counterpart, played by series veteran Chandler Riggs, was famously killed off midway through Season Eight when he shot himself before he could succumb to an incurable walker bite. Executive producer and former five-season showrunner Scott Gimple said previously the show still intends to adapt Carl's post-war storylines, which would be divvied up and given to other still-surviving characters on the show.
"We're committed to keep telling the story from the comic book. Yes, it's going to be different. And yes, Negan's relationship to Carl was a big part of the story moving forward, especially in the world changes we see early on. But, as usual, we do plan on telling those comic stories," Gimple told EW.
"It just will have to be with different people and it's a different way to tell those same stories. But we hope to still fulfill what those stories do."
Following the conclusion of the All Out War story arc and the launch of A New Beginning — taking place roughly two years later — two of Carl's biggest stories in his ongoing comic book journey were his relationship with Lydia and his dynamic with an imprisoned Negan, then housed in the Alexandria jail.
The Walking Dead will undergo an 18-month time jump when the series returns this October, bringing forth a new status quo where Hilltop leader Maggie (Lauren Cohan) has given birth to a son and Negan (Jeffrey Dean Morgan) remains a bearded Alexandrian prisoner.
Should another time skip follow, Lydia could then act as romantic interest for young Kingdom member Henry (Macsen Lintz), a show-only creation who could take over some of Carl's newly-available comic book storylines.