|This article centers around Derek's Killer. You may be looking for Sutton's Killer (Book Character) .|
Derek's Killer is a anonymous character that killed Derek Rogers and Theresa Lopez, and may possibly know about Emma and Sutton.
After Sutton's disappearance in East Of Emma, Emma receives a note telling her to be Sutton or she's next in O Twin, Where Art Thou?. After Ethan and Sutton confront Derek asking about the note, Derek had no idea what they were talking about, Derek was found dead the next morning.
- Rebecca Sewell - She's been planning to get anything and everything out of her way to make her family whole again, and killing Derek may have been part of her plan.
- Thayer Rybak - At the end of the finale, the same exact weapon that was used to kill Theresa was revealed by him to be in his couch
- Alec Rybak - He wanted the secret of the twins quiet to protect Ted and himself. He could have committed the crime and is just working with someone on this like Thayer.
- Jordan Lyle - It is believed that he killed Derek and kidnapped Theresa because of the secrets revealed in "Regrets Only". He was about to tell Mads the mistakes he had done in the past but decides not to. And it is exposed in the episode "Bride and Go Seek" that he's not the culprit because the only thing that Rebecca asked him to do was to frame Alec. He is a murderer, but he killed his brother.
- Ted Mercer - It is believed that Ted paid Derek the twenty thousand dollars to keep him quiet, but got tired and killed him to make sure the secret is not exposed. In "Bride & Go Seek", Emma discovered Ted looking for his Hospital ID badge which he thought he left at Derek's grave when he had left flowers. It is highly possible that he was the one to kidnap Theresa. And in the last part of "Bride and Go Seek", the guilty killer, which may have been Ted himself, visited Derek's grave looking for his work badge and Emma confronts him there with the badge. It had been revealed that Ted left flowers for Derek because he felt sorry because of Derek being so young.