"She deep down knows that this isn't someone that Jack would have approved of," Hannah Zeile tells PEOPLE of her character Kate Pearson

As Jack (Milo Ventimiglia) wisely told his daughter in the past, “even things that we like can be bad for us, sweetheart.” And This Is Us fans should be worried: Our Katie Girl may be headed for trouble with her boyfriend Marc (Austin Abrams).

Audiences watched as adult Kate (Chrissy Metz) had one hell of a week due to increasingly stressful marital woes with Toby (Chris Sullivan), who skipped the family retreat for their blind son Jack. Meanwhile, viewers learned more about the formative years of teen Kate (Hannah Zeile)’s relationship with her record store co-worker, and there’s not much to like about Marc.

Not only did he body-shame her when she reached for his box of chocolates and was unable to control his rage in front of Rebecca (Mandy Moore) during their coffee meet-up, but he also kicked Kate out of his car mid-drive to the Pearson cabin and left her stranded, cold and alone. Before he eventually found her and took her to the cabin, Kate frantically called her mother, who rushed to the cabin with sons Kevin and Randall, fearing for her daughter’s safety.

Before we find out what went wrong with Kate and Marc next week, Zeile tells PEOPLE that fans have only seen the beginning of just how badly Kate was affected by her first love Marc — and subsequently how that may impact her current relationship with Toby.

“I know that being in a toxic relationship can really just make you blame yourself at times when it’s not actually you, it’s the other person,” the 22-year-old actress says. “Marc’s own insecurities about himself are why he’s controlling. He’s projecting onto Kate.”

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It’s telling how different Marc is from Jack, who was the ideal male figure in Kate’s life as seen by the fairytale bedtime story, during which she was able to speak up and create her own destiny. But Jack’s death changed the course of her life, something Zeile calls “heartbreaking and definitely painful.” Sadly, Kate has become more closed off with her emotions since his tragic passing.

“Not having the person you really counted on and leaned on when you were younger really, really shook the rest of her adult life. It’s been interesting seeing the storyline with Toby and adult Kate where you can see her having those feelings of self-doubt. Again, he’s gotten into shape and he’s met this group at CrossFit where he’s obviously working out with men and other women who are probably in good shape. You can see her start to feel threatened by that, and that probably does route back to the feelings that she had in her first relationship,” Zeile explains.

“The issue everyone focuses on is her weight. The weight is really the collateral damage from the actual problems which were losing your father, being in an abusive relationship and feeling put down because of that,” she continues. “That’s when people turn to coping mechanisms such as overeating or different things. Although it’s really painful, it’s important to show that being overweight is not all that there is to focus on Kate. There are reasons behind that. Peeling back the layers, getting into why and how this all came to be, I think is, although sad, very rewarding and interesting for the viewers. It gives you more of a full picture of the character.”

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Credit: Ron Batzdorff/NBC

Describing Kate and Marc’s love affair as “abusive,” the star teases that her character goes through a lot of ups and downs (perhaps more of the latter). “Abusive doesn’t always have to be physical at all. I think that there’s emotional, verbal and mental, there are all types of abuse,” she says. “I don’t really categorize one above the other, but we can definitely see in this episode some verbal and mental abuse that Kate goes through.”

Furthermore, Zeile reminds fans to recall adult Kate and Rebecca’s discovery of throwback photos from a past family dinner during which Marc showed up unannounced. “I wanted to believe so badly that you kids were happy, I didn’t see what was happening,” the matriarch said, to which Kate unnervingly responded: “I didn’t see it either.”

“That’s 25 years later and Kate’s reaction is still so visceral. You can see the pain that she and Rebecca share. So obviously this whole experience is an important landmark in her life. I think that has a lot to do with it being her first boyfriend. Her first experience, her first definition of what she thinks love is, is with him,” Zeile says.

And though everyone does not know a lot about Marc, Zeile agrees with fans that he does have an ominous quality to be wary of. “There have definitely been some subtle hints here and there that aren’t so in your face, but a little bit of a warning sign. For example, him coming to dinner unannounced by getting her address off her resumé is a little invasive,” she admits. “But also the writers do a good job of making him human, he does have a likeability and a charm. Sometimes he does seem sweet, and that’s how he reels Kate back in. I do think he’s broken, but that doesn’t mean that it’s Kate’s job to fix him.”

So if Jack were still alive, would Kate be dating a guy like Marc? Zeile thinks not.

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“Jack was a very honorable man and he set the bar really high for what Kate should expect of a man. Maybe that’s part of the reason that she doesn’t really open up to her family at first about any struggles going on with Marc and is a little to herself, because maybe she deep down knows that this isn’t someone that Jack would have approved of,” she predicts.

“Her relationship with her father was so close, he was her rock. Then she loses her father and clings on to Marc, who was the first person that she’s ever felt like looked at her the way that her father looked at her,” she says. “She doesn’t have the best self-confidence, so she really grasps the attention that she gets from Marc. I think Marc is the first person to try and fill that void for her. She so badly just wants this to work that she is a little blinded by the good.”

When it comes to Marc’s rage-filled reactions, Zeile says Kate is “choosing not to see the bad,” adding, “Sometimes you want to protect the other person because you love them. You keep their secrets for them and you don’t want to disclose to other people because then you’re kind of pulling back the curtain. You shouldn’t feel ashamed about being mistreated. It’s not anything that you’re doing. It’s not your fault.”

As for what’s to come next for Kate and Marc, Zeile says “everything really comes to fruition” in the episode following The Big 3 trilogy. “I think those three episodes have really been a lead-up and it’s all going to unfold. The show is like a giant puzzle. We’re filling in the blanks, slowly piece by piece. We know where the end result is, but what led there — obviously the death of her father and then having an abusive relationship — her personal self-confidence probably hindered some of her own opportunities. This show always had that element of how one decision can change the trajectory of your life.”

This Is Us airs Tuesdays (at 9 p.m. ET) on NBC.