Warning: Major Spoilers Ahead for One of Us Is Lying.
One of Us Is Lying concludes with Simon's murder solved but leaves a major cliffhanger in the form of a mysterious new blackmailer who threatens the main cast over text. The Peacock original series, based on Karen M. McManus's novel, follows a group of four students as they try to exonerate themselves from the murder of their classmate, Simon Kelleher (Mark McKenna). But while the book ends rather definitively, it seems the series may be preparing for a second season.
At the end of One of Us Is Lying season 1, Nate (Cooper van Grootel), Cooper (Chibuikem Uche), Bronwyn (Marianly Tejada), and Addy (Annalisa Cochrane), learn that Addy's ex-boyfriend Jake is the real killer. Angry with Addy for cheating on him and bitter towards Simon for threatening to write about it on his gossip app, Jake concocts an elaborate plan in which he and Simon will "prank" the four by giving Simon a severe allergic reaction while he's in detention with them. Simon agrees to the plan on the assumption that he'll be saved by an EpiPen, but Jake secretly takes the spare EpiPens from the nurse's office to ensure Simon will die (this marks a major change to Simon's death in the book).
Season 1 delivers another shocking change from the book when Jake is accidentally shot and killed during an altercation with the Bayview four. The four then presumably hide the body and agree to cover the whole thing up. But at the end of the finale episode, each member of the group gets a text saying "DO AS I TELL YOU. OR EVERYONE WILL KNOW WHAT YOU DID," along with incriminating photos. This final twist of One of Us Is Lying season 1 seems to indicate that the group will have a new mystery to solve in a potential season 2. Of the characters the show has already introduced, here are the ones who could have reason to blackmail the main cast.
Vanessa
Throughout One of Us Is Lying, Vanessa acts like she's auditioning for a 2021 remake of Mean Girls. She loves digging up dirt on her classmates and putting everyone down, especially the Bayview four. This alone provides a reasonable motive for Vanessa to spy on the group and try to take them down. However, she has an even stronger one as well. During the group's investigation of Ms. Avery, they learn that the teacher helped Vanessa get an abortion and hide it from her parents. When they suspect Ms. Avery killed Simon, the four go to the police with this information, meaning Vanessa's secret is out and she must reckon with her parents' reaction. Understandably, this leaves Vanessa furious, and would definitely provide the necessary motivation for her to seek revenge by blackmailing them using evidence of Jake's death.
Ms. Avery
Ms. Avery may not be the worst teacher at Bayview High, but she certainly seems capable of blackmail. Ms. Avery herself was blackmailed by Simon when he found out that she helped Vanessa get an abortion, and once Addy, Bronwyn, Nate, and Cooper reveal this information to the police, Ms. Avery loses her job and her entire career. This would be a devastating blow to the teacher, and while she may not have any stake in what happens to Jake and the mystery surrounding Simon's death, she's angry enough at the students who cost her her life's work to try to ruin their lives, as well. It's unclear what she would want the four to do for her in exchange for her silence since they won't be able to get her job back, but it's possible Ms. Avery would be satisfied enough with making them sweat.
TJ
A staple teen drama trope is the guy who steals his best friend's girl, and TJ initially fits this role perfectly in One of Us Is Lying. However, TJ never succeeds in winning Addy over from Jake, and she even gets him suspended from school by lying. The four continue to make TJ's life miserable by harassing him about Simon's death and stealing his phone at homecoming, so it's reasonable to assume he hates the Bayview four by the end of season 1. On top of this, he and Jake have been best friends for a long time, so in addition to wanting the four to pay, TJ could even be looking for justice for Jake. However, TJ also makes it clear to Addy in episode 6, "One of Us Is Dancing!" that he wants nothing more to do with her, and getting involved with Jake's death would mean getting involved with Addy again.
Evan
One of Us Is Lying's high school stereotypes don't end with TJ. Evan, Bronwyn's boyfriend for part of the season, is a debate team star and devoted academic like her. He encourages Bronwyn not to get tangled up with the other murder suspects, but she continually ignores his advice and they end up breaking up. Evan may not seem like the revenge type, but seeing Bronwyn and Nate's budding romance could add enough fuel to his anger to make him blackmail the group. It's unclear exactly what Evan would gain by doing this since he doesn't seem to deal in high school gossip much, but with his debate skills, he could easily be thinking several steps ahead.
Maeve
Maeve, Bronwyn's sister, is probably One of Us Is Lying's most intelligent main character. Throughout season 1 she usually offers help to the Bayview four, but Bronwyn often pushes her away. It seems somewhat unlikely to think Maeve would blackmail her own sister, especially since she's no fan of Jake, but the fact that she leaves the cafeteria with a dirty look at her sister and Janae right before the texts are sent is too suspicious to discount. And unlike any of the other blackmail suspects, Maeve has as much reason to keep the secret as to reveal it. While Vanessa, Ms. Avery, TJ, and Evan would be primarily motivated by revenge, Maeve could simply be looking for attention, which would make her an even more dangerous blackmailer. Her tech skills would also explain how she was able to hack into the "SIMON SAYS" server Jake created in order to send the texts.
Season 1's ending explained the murder mystery, but One of Us Is Lying has left the door wide open for season 2 with the blackmailer twist. While any of these suspects could be the blackmailer, it's also possible that One of Us Is Lying season 2 could introduce new characters if the show is renewed. The book has a sequel, called One of Us Is Next, which could hold some answers for the series' next season even though the show has already made some significant plot departures. Either way, if One of Us Is Lying does get a season 2, the group will have to figure out who the blackmailer is before their dirty secret gets out.