While we know the truth, the Liars don't, and since the audience isn't able to reach through the TV and drop them a note, they're going to be in the dark for a bit. However, we do know his secret won't be kept forever.
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But which Little Liar will be the first to learn the truth? We're going to speculate about why each Liar might be the first to know, and think about whether or not we want them to have that pivotal role, starting with Emily (Shay Mitchell).
Why she might be the first to know: After Spencer (Troian Bellisario), Emily is the Liar that is closest to Toby. They've been friends for a long time, and he clearly feels like he can trust her. If Toby is infiltrating the "A" team with good intentions, he might open up to Emily about it. He even might go to her if he's had bad intentions in the past, but is starting to feel conflicted or wants to get out of the team but doesn't know how.
More likely, the discovery of his secret will be accidental. But, again, Emily's friendship with Toby makes her a likely candidate for stumbling on his secret, since they spend time together, and he might even invite her to his house to hang out.
Why this would be interesting: Because Emily is close to Toby, his betrayal would be personal — but it wouldn't completely break her (or make her act crazy) like it might Spencer. Also, it would be interesting to see how she dealt with it. Would she tell Paige (Lindsey Shaw) first, or the other Liars? Spencer has attacked many of Emily's friends and love interests in the past — including, Toby back in Season 1 — so it would be ironic if Em ended up being the one who knows it's Spencer's boyfriend who's the baddie.
Why we don't want it. On the other hand, hasn't poor Em been through enough? It would be nice to see the writers go a tad easier on her this winter. And we'd love to see her mend her relationship with Spencer fully after the hurt caused by Spencer accusing Paige, and this would probably only tear them apart.
What do you think: Should Emily be the one to find out first?