Who is Polly Cooper? "Riverdale" has made it clear she is Betty's sister, but not much is known about Polly beyond that. Starting in episode six, though, that will all change. During a set visit, Lili Reinhart -- who plays girl-next-door Betty Cooper -- spoke to CBR and other reporters about Polly's debut, Polly's relationship with Betty and why Betty may be a little stronger than her sister.

"Once Polly is introduced and you get to know who she is as a character rather than just this person that everyone talks about that no one really sees, you see Betty and Polly together and you instantly can see why Betty was fighting so hard to find out the truth about what happened to Polly because they're just incredibly close," Reinhart revealed.

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"I hate to say Betty is stronger than Polly," she added. "Betty is the younger sister who takes the role of the older sister between the two of them, in the sense that she doesn't let her parents manipulate her the way that Polly let her parents manipulate her. You do get to know Polly and you get to see their beautiful sister relationship, and how Polly cares just as much about Betty as Betty cares about Polly."

Reinhart also addressed Betty's breakdown in episode three, where she went a little overboard when punishing Chuck for the Sticky Maple incident. "She cares about her sister so much, and the fact that something so terrible happened to her sister just causes her to kind of break down and go into defense mode," she explained. "Betty has a lot of built-up anxiety and anger and frustration. Tonight's episode is kind of the boiling over point and that's why you see her alter ego. I would say it's more of an alter ego than a split personality. It's her bad side coming out."

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According to the synopsis for "Faster, Pussycats! Kill! Kill!", the episode in which Polly debuts,

With Riverdale High’s annual variety show around the corner, Valerie’s (guest star Hayley Law) efforts to help Archie (KJ Apa) prepare for his big performance lead to some major fallout between her and Josie (Ashleigh Murray).

Meanwhile, Betty (Lili Reinhart) and Jughead’s (Cole Sprouse) investigation into Jason’s death brings them to a home for troubled youth and face-to-face with Polly (guest star Tiera Skovbye).

Just as her relationship with her mother was on the upswing, an unexpected betrayal sends Veronica (Camila Mendes) spiraling, while Josie deals with her own pressures at home after her overbearing father returns for her performance at the variety show.

Finally, Hermoine’s (Marisol Nichols) first day at Andrew’s Construction finds her and Fred (Luke Perry) growing closer.

Starring K.J. Apa as Archie Andrews, “Riverdale” airs Thursdays at 9 pm on The CW. The series also stars Cole Sprouse as Jughead Jones, Camila Mendes as Veronica Lodge, Lili Reinhart as Betty Cooper, Ross Butler as Reggie Mantle and more. “Breakfast Club” alum Molly Ringwald will also recur as Archie’s mother Mary.