Theo Cotter's only appearance is in Book 4 . He is the grandson of Hillary Vale , an old friend of Grace 's whom she used to visit every year. Grace left nothing to Theo and Hillary in her will and this upset them because they were becoming poorer and poorer. They attempt to steal the statue of Sakhet from Dan and Amy because Irina tells Theo that she will pay him alot of money for it. They are caught by Amy and Dan. the siblings forced him to double cross Irina and give her a fake statue. Irina takes the fake statue and tells Theo that it's a fake statue (but in her mind it's the real one) and takes off without giving him the money. Throughout Book 4 , Nellie likes Theo but hates him in the end. She got over him in about 5 minutes. He is briefly mentioned in The Black Book of Buried Secrets, when Nellie commented that Theo's accent was awesome.
He had two failed attempts to steal the Sakhet. The first one was when Hillary tried to cut out the waist pack Amy was wearing until Dan saved her. His second failed attempt was when Amy was asleep, he was tackled by Nellie Gomez.