This Is Us Theory That "Her" Is 2 Different People
This Theory About "Her" on This Is Us Is a Total Game Changer
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Marty5151 posits that This Is Us is planning a major twist that involves the "her" in question being Zoe, who will be responsible for hurting Kevin in a serious way. This would certainly explain why Tess is avoiding seeing this person since she's been shown to be close with her Uncle Kevin. And it would also make sense that Toby would need to be there to support Kate through possibly saying goodbye to Kevin, even if they're separated.
Zoe isn't the only option though. There's an excellent chance that Toby's "her" is Kate, while Randall, Tess, and the rest of the Pearsons are all going to say farewell to Rebecca. It seems odd that Randall is the one rallying everyone together if Rebecca is still alive and well. However, if she's been suffering from a prolonged illness, it would be natural for Tess to be having trouble working up the nerve to visit her grandmother, and it would also be totally normal for Toby to be unsure whether he should go see Rebecca at all if he's on the outs with Kate.
No matter who the original "her" is, there's plenty of evidence to suggest that there are actually two different mystery women now, thanks to Randall and Toby's phone call. It seems clear at this point that one of those women is Kate, but don't be surprised is she's doesn't turn out to be the one that has the entire Pearson clan gathering together.