Role in the novel (contains several spoilers) Edit
She later turns out to be in league with Mike. The two had met a few years prior to the opening of the story in Germany. No one knew this apart from Andrew Lipincott. Greta had the idea to make Mike a rich man by introducing him to Ellie. They later make a plan to kill Ellie with cyanide. The plan went fairly well except for the fact that another friend of Ellie, Claudia, had accidentally taken the cyanide laced pills. In that case, the smell of cyanide was still there and led the police to believe that Ellie had been poisoned. Mike later strangles Greta after confessing to her that he really loved Ellie.
In Endless Night's 1972 movie adaptation, Greta Andersen was portrayed by Britt Ekland.